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/EnsembleFileSet/RimEnsembleFileSetCollection.h

@@ -18,10 +18,10 @@
 
 #pragma once
 
-#include "RimEnsembleFileSet.h"
-
 #include "cafPdmObjectCollection.h"
 
+class RimEnsembleFileSet;
+
 //==================================================================================================
 ///
 /// Class to manage a collection of ensemble filesets