Storing meta data in pmd structure

[daico007]

PR Summary:

Relate to #495. Start storing meta data like combining rule and scaling factors in parametrized parmed structure. Currently utilizing the _Defaults class from parmed.gromacs.gromacstop so terminology may be GROMACS specific, but the info can be stored and access there.

PR Checklist

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• [ ] Includes appropriate unit test(s)
• [ ] Appropriate docstring(s) are added/updated
• [ ] Code is (approximately) PEP8 compliant
• [ ] Issue(s) raised/addressed?