made plasma optional for export h5m with pymoab

[shimwell]

Proposed changes

This PR adds a flag for including the plasma or not in the PyMoab

As requested in issue #824

@billingsley-john could you take a look and add a test to check this works as you would like

Types of changes

What types of changes does your code introduce to the Paramak?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation Update (if none of the other choices apply)
• New tests

Checklist

• Pep8 applied
• Unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)