Refactor: forward-declare element type in collection headers

Move the element-type include from each migrated collection header into
its .cpp, leaving only a forward declaration in the header. Follows the
pattern from 9e4d79159 for RimJobCollection, made possible by the
constructor-time PdmObject base check.

ApplicationLibCode/ProjectDataModel/GeoMech/RimGeoMechContourMapViewCollection.cpp

@@ -1,6 +1,7 @@
 #include "RimGeoMechContourMapViewCollection.h"
 
 #include "RimCase.h"
+#include "RimGeoMechContourMapView.h"
 
 CAF_PDM_SOURCE_INIT( RimGeoMechContourMapViewCollection, "GeoMech2dViewCollection" );