0026859: Export of GDT from XCAF to STEP

Export/Import:
Add export of dimensions, geometric tolerances and datums, according to AP242.
Add missing STEP entities.
Corrections.

XCAF:
Add new attributes to Datum object: DatumTargetNumber.
Supplement enum XCAFDimTolObjects_GeomToleranceZoneModif.
Corrections.

Test cases:
gdt export A1
gdt export A2
gdt export A3
gdt export A4
gdt export A5

src/StepDimTol/StepDimTol_SimpleDatumReferenceModifierMember.hxx

@@ -44,6 +44,9 @@ public:
 
   virtual Standard_Boolean SetName(const Standard_CString /*theName*/) Standard_OVERRIDE
     {  return Standard_True;  }
+
+  inline Standard_Integer Kind() const
+    {return 4;}
   
   Standard_EXPORT virtual   Standard_CString EnumText()  const Standard_OVERRIDE;