+ 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/App/Cell.cpp

@@ -235,7 +235,7 @@ void Cell::setContent(const char * value)
                     if (expr)
                         delete expr->eval();
                 }
-                catch (Base::Exception & e) {
+                catch (Base::Exception &) {
                     expr = new StringExpression(owner->sheet(), value);
                 }
             }