Garbage Collection in del function, Updated union of lists function

[vijay-jaisankar]

In this PR, the following changes have been implemented:

• Updated the unionSequence function in ocl.py
• Added an explicit call to the garbage collection module in the free function of oclfile.py; this should make things a little more efficient.

[Zishan-Rahman]

@vijay-jaisankar (sorry for the ping) The garbage collection code is perfectly fine to merge. However, the unionSequence function is meant to contain duplicates. This mirrors how the union function in Acceleo's implementation of OCL, according to the UML standard, behaves with Sequences. For example, Sequence{'bbb', 'b'}->union({'b', 'bb'}) should evaluate to Sequence{'bbb', 'b', 'b', 'bb'}. If you still want this merged, please can you undo the unionSequence modification and push your changes here so that @kevinlano can merge them (I'm one of his PhD students)?

[kevinlano]

Change to oclfile.py is approved, but not to ocl.py

[vijay-jaisankar]

Hello, thanks for the information about the implementation of the OCL. Sure, I can make another commit to this PR if you'd like, reverting the change to the mentioned file.

[Zishan-Rahman]

Hello, thanks for the information about the implementation of the OCL. Sure, I can make another commit to this PR if you'd like, reverting the change to the mentioned file.

@vijay-jaisankar Looks like @kevinlano has already merged this PR just now, so feel free to create another PR reverting the ocl.py union changes.