DONOTMERGE: Purge communicator before free on mpich4+

[vehre]

coverage on master

Summary of changes

Purge all pending messages from the communicator before freeing it.

Rationale for changes

MPICH 4+ had strange errors when freeing a communicator that had dangling messages. This patch removes all messages before freeing the communicator.

Additional info and certifications

This pull request (PR) is a:

• Bug fix
• Feature addition
• Other, Please describe:

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    be introduced)
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  • I have you written new tests for these changes
  • I have successfully tested these changes locally
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  • The commits are logically atomic, self consistent and coherent
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  • Test coverage is maintained or increased after this is merged

Code coverage data

[vehre]

The initial version of the patch had no guard for MPICH. The fix is only needed an valid for MPICH. So that has been added.

[vehre]

I have to rethink this. Does not work as expected. Closing.