0024023: Revamp the OCCT Handle - gcc and clang

Adaptations for compiling with GCC 4.7 and 4.8:
- Construction semantics is used for Handle objects being initialized by const Handle objects of derived type, to avoid overload resolution error in GCC 4.7.
- Missing includes added.
- Fixed bugs related to misuse of direct casts of handle.
- Eliminate CLang warnings on uninitialized and unused variables, functions, and expressions

src/STEPConstruct/STEPConstruct_Assembly.cxx

@@ -143,7 +143,7 @@ void STEPConstruct_Assembly::MakeRelationship ()
 
 Handle(Standard_Transient)  STEPConstruct_Assembly::ItemValue () const
 {
-  if (theval.IsNull()) return thesr;
+  if (theval.IsNull()) return Handle(Standard_Transient) (thesr);
   return theval;
 }
 

src/STEPConstruct/STEPConstruct_ContextTool.cxx

@@ -43,6 +43,7 @@
 #include <STEPConstruct_Part.hxx>
 #include <StepData_StepModel.hxx>
 #include <StepGeom_Axis2Placement3d.hxx>
+#include <StepRepr_NextAssemblyUsageOccurrence.hxx>
 #include <StepRepr_PropertyDefinition.hxx>
 #include <StepShape_ShapeDefinitionRepresentation.hxx>
 #include <TCollection_HAsciiString.hxx>