[RF] Don't ignore ParamSetting values in HistFactory Asimov dataset

HistFactory allows to set nuisance parameters to given values (and
possibly set them to constant) by using something like

```
<ParamSetting Val=1 Const="True"> myNP </ParamSetting>
```

However the Asimov dataset created by `hist2workspace` does not respect
this setting, because in
[MakeCombiendModel](https://root.cern/doc/master/HistoToWorkspaceFactoryFast_8cxx_source.html#l02109)
the Asimov workspace is created before the ParamSettings are applied.

This commit is fixing this problem.

The problem can be reproduced with the following script/instructions:

```C++
using namespace RooStats;
using namespace HistFactory;

// Step 1: initialize the XML files
//
// Run prepareHistFactory before in this directory
//
// Modify the GaussExample in the generated example to be:
//
// <Measurement Name="GaussExample" Lumi="1." LumiRelErr="0.1" >
//   <POI>SigXsecOverSM</POI>
//   <ParamSetting Const="True">Lumi</ParamSetting>
//   <ParamSetting Val="20" Const="True">alpha_syst1</ParamSetting>
// </Measurement>

// Step 2: Create the RooWorkspace
//

ConfigParser parser{};
auto measurements = parser.GetMeasurementsFromXML("config/example.xml");

// Get the first example, the GaussExample
Measurement& meas = measurements.front();

// Collect the histograms from their files,
meas.CollectHistograms();

// Now, do the measurement
std::unique_ptr<RooWorkspace> ws{MakeModelAndMeasurementFast( meas )};

// Step 3: Validation plot to show mismatch between model and Asimov data
//

auto x = ws->var("obs_x_channel1");
auto data = ws->data("asimovData");
auto pdf = ws->pdf("model_channel1");

// Draw the asimov data together with the model and check if they match
TCanvas c1("c1");
RooPlot *xframe = x->frame();
data->plotOn(xframe);
pdf->plotOn(xframe);

xframe->Draw();

c1.SaveAs("plot.png");

// Note that the errors will be plotted wrongly if the Asimov data is
// correct. That's because if the changed alpha_syst1 value is considered
// correctly, the asimov dataset has non-integer weights. In this case, the
// plotting uses SumW2 errors instead of Poisson erros, and the weights
// squared is not filled correctly in the Asimov data that is created by
// AsymptoticCalculator::GenerateAsimovData(). But this is a completely
// different issue.
```

Author: guitargeek

roofit/histfactory/inc/RooStats/HistFactory/HistoToWorkspaceFactoryFast.h

@@ -100,7 +100,7 @@ namespace RooStats{
             std::vector<std::unique_ptr<RooWorkspace>>& wspace_vec,
             std::vector<std::string> const& channel_names,
             std::string const& dataSetName,
-            RooArgList obsList,
+            RooArgList const& obsList,
             RooCategory* channelCat);
 
       RooHistFunc* MakeExpectedHistFunc(const TH1* hist, RooWorkspace* proto, std::string prefix,

roofit/histfactory/src/HistoToWorkspaceFactoryFast.cxx

@@ -1872,55 +1872,29 @@ RooArgList HistoToWorkspaceFactoryFast::createObservables(const TH1 *hist, RooWo
     combined->defineSet("globalObservables",globalObs);
     combined_config->SetGlobalObservables(*combined->set("globalObservables"));
 
-
-    ////////////////////////////////////////////
-    // Make toy simultaneous dataset
-    cxcoutP(HistFactory) << "\n-----------------------------------------\n"
-        << "\tcreate toy data for " << channelString.str()
-        << "\n-----------------------------------------\n" << endl;
-
-
-    // now with weighted datasets
-    // First Asimov
-    //RooDataSet * simData=nullptr;
     combined->factory("weightVar[0,-1e10,1e10]");
     obsList.add(*combined->var("weightVar"));
+    combined->defineSet("observables",{obsList, *channelCat}, /*importMissing=*/true);
+    combined_config->SetObservables(*combined->set("observables"));
 
-    // Create Asimov data for the combined dataset
-    std::unique_ptr<RooDataSet> asimov_combined{static_cast<RooDataSet*>(AsymptoticCalculator::GenerateAsimovData(*simPdf,
-                                  obsList))};
-    if( asimov_combined ) {
-      combined->import( *asimov_combined, Rename("asimovData"));
-    }
-    else {
-      std::cout << "Error: Failed to create combined asimov dataset" << std::endl;
-      throw hf_exc();
-    }
 
     // Now merge the observable datasets across the channels
     for(RooAbsData * data : chs[0]->allData()) {
-      // Only include RooDataSets where a data with the same name doesn't only
-      // exist in the merged workspace (like the "asimovData").
-      if(dynamic_cast<RooDataSet*>(data) && !combined->data(data->GetName())) {
+      // We are excluding the Asimov data, because it needs to be regenerated
+      // later after the parameter values are set.
+      if(std::string("asimovData") != data->GetName()) {
         MergeDataSets(combined, chs, ch_names, data->GetName(), obsList, channelCat);
       }
     }
 
 
-    obsList.add(*channelCat);
-    combined->defineSet("observables",obsList);
-    combined_config->SetObservables(*combined->set("observables"));
-
     if (RooMsgService::instance().isActive(static_cast<TObject*>(nullptr), RooFit::HistFactory, RooFit::INFO))
       combined->Print();
 
     cxcoutP(HistFactory) << "\n-----------------------------------------\n"
             << "\tImporting combined model"
             << "\n-----------------------------------------\n" << endl;
     combined->import(*simPdf,RecycleConflictNodes());
-    //combined->import(*simPdf, RenameVariable("SigXsecOverSM","SigXsecOverSM_comb"));
-    // cout << "check pointer " << simPdf << endl;
-    //    cout << "check val " << simPdf->getVal() << endl;
 
     std::map< std::string, double>::iterator param_itr = fParamValues.begin();
     for( ; param_itr != fParamValues.end(); ++param_itr ){
@@ -1957,6 +1931,29 @@ RooArgList HistoToWorkspaceFactoryFast::createObservables(const TH1 *hist, RooWo
     combined->importClassCode();
     //    combined->writeToFile("results/model_combined.root");
 
+
+    ////////////////////////////////////////////
+    // Make toy simultaneous dataset
+    cxcoutP(HistFactory) << "\n-----------------------------------------\n"
+        << "\tcreate toy data for " << channelString.str()
+        << "\n-----------------------------------------\n" << endl;
+
+
+    // now with weighted datasets
+    // First Asimov
+
+    // Create Asimov data for the combined dataset
+    std::unique_ptr<RooDataSet> asimov_combined{static_cast<RooDataSet*>(AsymptoticCalculator::GenerateAsimovData(
+                                  *combined->pdf("simPdf"),
+                                  obsList))};
+    if( asimov_combined ) {
+      combined->import( *asimov_combined, Rename("asimovData"));
+    }
+    else {
+      std::cout << "Error: Failed to create combined asimov dataset" << std::endl;
+      throw hf_exc();
+    }
+
     return combined;
   }
 
@@ -1965,7 +1962,7 @@ RooArgList HistoToWorkspaceFactoryFast::createObservables(const TH1 *hist, RooWo
                       std::vector<std::unique_ptr<RooWorkspace>>& wspace_vec,
                       std::vector<std::string> const& channel_names,
                       std::string const& dataSetName,
-                      RooArgList obsList,
+                      RooArgList const& obsList,
                       RooCategory* channelCat) {
 
     // Create the total dataset