Add smiles string conversion using pybel backend

[daico007]

PR Summary:

A quick path to go from mbuild compound to smiles string through openbabel. I am using openbabel to guess the bond orders of the resulted molecule and output a smile string. This can be a temp solution before we can have our own smiles string writer. Pending unit tests.

PR Checklist

---

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.66%. Comparing base (fd3fe39) to head (efd129f).
Report is 146 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1056      +/-   ##
==========================================
- Coverage   90.53%   88.66%   -1.88%     
==========================================
  Files          60       60              
  Lines        8421     8423       +2     
==========================================
- Hits         7624     7468     -156     
- Misses        797      955     +158     

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[bc118]

Fix bleeding test, otherwise this LGTM

[bc118]

LGTM after fixing bleeding test

[daico007]

So the issue with the bleeding test has to do with the python version specified. When updating the env with specs from foyer/gmso env.yml, the python version got updated to python 3.10. The fixes would need to be applied on gmso/foyer sides, by adding an upper constraint for python version