Add new default option for Compound.save()

[daico007]

PR Summary:

Address #1152. Add new default option for Compound.save() (None), which will use combining rule from XML.

PR Checklist

---

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

[CalCraven]

LGTM! Thanks Co

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (b30ceff) 87.19% compared to head (a5ec3e2) 87.18%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1153      +/-   ##
==========================================
- Coverage   87.19%   87.18%   -0.02%     
==========================================
  Files          62       62              
  Lines        6490     6492       +2     
==========================================
+ Hits         5659     5660       +1     
- Misses        831      832       +1     

Files	Coverage Δ
mbuild/conversion.py	89.69% <50.00%> (-0.12%)	⬇️

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