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/UnitTests/ListKeywordsForObjectsAndFields-Test.cpp

@@ -92,9 +92,7 @@ TEST( ListKeywords, ListAllObjectKeywordsAndFieldKeywords )
 
         stream << "\n";
 
-        std::vector<caf::PdmFieldHandle*> fields;
-        myClass->fields( fields );
-
+        std::vector<caf::PdmFieldHandle*> fields = myClass->fields();
         for ( auto f : fields )
         {
             stream << "  " << f->keyword() << "\n";