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[bot]]

Codecov Report

Base: 90.53% // Head: 88.66% // Decreases project coverage by -1.87% :warning:

Coverage data is based on head (efd129f) compared to base (fd3fe39). Patch coverage: 100.00% of modified lines in pull request are covered.

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     

Impacted Files	Coverage Δ
mbuild/compound.py	97.58% <100.00%> (+<0.01%)	:arrow_up:
mbuild/formats/hoomd_forcefield.py	0.00% <0.00%> (-78.46%)	:arrow_down:
mbuild/formats/hoomd_snapshot.py	73.77% <0.00%> (-5.25%)	:arrow_down:

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[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