+ Fixed merge of cells to retain value in upper left cell+ Replaced std::map::at() with iterators and find. Removed some superfluous calls to std::map::find()

+ Visual Studio fixes
+ Fixed return values
+ Removed unused variables

src/Mod/Spreadsheet/Gui/SheetModel.cpp

@@ -355,6 +355,8 @@ QVariant SheetModel::data(const QModelIndex &index, int role) const
             return QVariant();
         }
     }
+
+    return QVariant();
 }
 
 QVariant SheetModel::headerData(int section, Qt::Orientation orientation, int role) const