Change API for PdmObjectHandle and PdmFieldHandle

* Refactor interface to PdmObjectHandle and PdmFieldHandle
Return objects instead of passing in structures as parameters

* Add nodiscard to several functions
* Remove redundant this->
* Rename to ptrReferencedObjectsByType

ApplicationLibCode/ProjectDataModel/RimPlot.h

@@ -90,8 +90,7 @@ public:
     template <typename PlotWindowType = RimPlotWindow>
     bool isSubPlot() const
     {
-        PlotWindowType* parentPlotWindow = nullptr;
-        firstAncestorOfType( parentPlotWindow );
+        auto parentPlotWindow = firstAncestorOfType<PlotWindowType>();
         return parentPlotWindow != nullptr;
     }