AssociationImporter Fix.

The Import Method of the AssociationImporter does not work.

Updated the "ImportAction.Insert" code to:
1) Fix Exception caused by line 124. "sequence contains more than one element" when multiple associations already defined in the product.
2) Prevent adding Duplicate Association Groups when importing.
3) Prevent adding Duplicate Associations to a Associatio Group.

The other sections of the case statement (InsertAndReplace,Update,Delete) do not work because the AssociationId is not Mapped from the import file.