Convert FTRL code to a core datatable class (#1417)

FTRL code has been converted into a core datatable class:
- added `fit` and `predict` methods for training and making predictions;
- added `reset` method to reset the model;
- added getters and setters for all the model parameters;
- the `model` itself can now be returned and loaded as a separate frame;
- some other refactoring and modifications.

The algo itself was also slightly optimized, however, it still gets the same score on Kaggle. 
So we're ok from the accuracy point of view.

Author: oleksiyskononenko

c/datatablemodule.cc

@@ -13,7 +13,7 @@
 #include "csv/writer.h"
 #include "expr/py_expr.h"
 #include "extras/aggregator.h"
-#include "extras/ftrl.h"
+#include "extras/py_ftrl.h"
 #include "frame/py_frame.h"
 #include "options.h"
 #include "py_column.h"
@@ -199,7 +199,6 @@ void DatatableModule::init_methods() {
   add(METHODv(expr_unaryop));
   add(METHOD0(is_debug_mode));
   add(METHOD0(has_omp_support));
-  init_methods_ftrl();
   init_methods_aggregate();
   init_methods_str();
   init_methods_options();
@@ -234,6 +233,8 @@ PyInit__datatable()
 
   try {
     py::Frame::Type::init(m);
+    py::Ftrl::Type::init(m);
+
   } catch (const std::exception& e) {
     exception_to_python(e);
     return nullptr;

c/datatablemodule.h

@@ -29,7 +29,6 @@ class DatatableModule : public py::ExtModule<DatatableModule> {
 
     void init_methods();
     void init_methods_aggregate();// extra/aggergate.cc
-    void init_methods_ftrl();     // extra/ftrl.cc
     void init_methods_str();      // str/py_str.cc
     void init_methods_options();  // options.cc
     void init_methods_sets();     // set_funcs.cc

c/extras/aggregator.cc

@@ -45,9 +45,9 @@ namespace py {
 
        // dt changes in-place with a new column added to the end of it
        DataTable* dt_members = agg.aggregate(dt).release();
-       py::Frame* frame_members = py::Frame::from_datatable(dt_members);
+       py::oobj df_members = py::oobj::from_new_reference(py::Frame::from_datatable(dt_members));
 
-       return frame_members;
+       return df_members;
      }
   );
 }