#20 Fix NGEN bugs for object callbacks on certain models

[Max Zinal]

This patch fixes the following NGEN bugs:

1. Duplicate 'case' sections multivalue attributes
which were generated in the elementSetAt(),
elemendAdded() and elementRemoved() functions.

2. Missed elementSetAt functions for objects with
multivalue attributes due to missing logic in
isNeedElementSetAt().

3. Added missing logic to assign object ownership to
attribute values in
elementSetAt(), elemendAdded() and elementRemoved()
generated functions. This problem could lead to
duplicate attribute value output (both in at the root
of the XMI.content and attribute).

4. Some minor code cleanup. Maybe not very critical,
but good to have.

[Max Zinal]

A patch for GenMMImplObject.java file