minor assimilation decay bug fix

bug fix- excessive assimilation decay because downstream length accumulated over time (Assimilate.cpp) [issue found by R.Chlumsky]

new warning if :SBGroupOverrideWeights is used for invalid blend (ParsePropertyFile.cpp) [issue found by R. Chlumsky]

Author: analytophile

src/Assimilate.cpp

@@ -142,7 +142,8 @@ void CModel::PrepareAssimilation(const optStruct &Options,const time_struct &tt)
 
   for(p=0; p<_nSubBasins; p++) {
     _aDAscale_last[p]=_aDAscale[p];
-    _aDADrainSum[p]=0.0;
+    _aDADrainSum  [p]=0.0;
+    _aDAlength    [p]=0.0;//reboot every timestep
   }
 
   // check for observations in each basin
@@ -192,6 +193,9 @@ void CModel::PrepareAssimilation(const optStruct &Options,const time_struct &tt)
         if(pdown!=DOESNT_EXIST) {
           _aDAlength   [p]+=_pSubBasins  [pdown]->GetReachLength(); //length propagates from below
         }
+        else{
+          _aDAlength[p]=0;
+        }
         _aDAtimesince[p]+=Options.timestep;
         _aDAoverride [p]=false;
         _aDAobsQ     [p]=0.0;

src/ParsePropertyFile.cpp

@@ -1306,6 +1306,10 @@ bool ParseClassPropertiesFile(CModel         *&pModel,
       if (Len >= 4) {
         // currently the N is the same for all subbasins. This may change in the future (?)
         int N = pModel->GetBlendedForcingsNumWeights(s[1]);
+        if (N==0){
+          WriteWarning("ParsePropertyFile: cannot use :SBGroupOverrideWeights if blending is not already in use. Command will be ignored.",Options.noisy);
+          break;
+        }
         double* uniform_nums = new double[N-1];
         double* wts          = new double[N];
         double  sum          = 0.0;