Fix inconsistent molecule fingerprint generation

[mliu49]

Motivation or Problem

The updated fingerprint approach resulted in different fingerprint depending on atom order when the molecule contained more than two elements aside from CHNOS.

This fixes #1740.

Description of Changes

This fixes the issue by sorting the elements alphabetically.

Testing

A unit test was written and the case outlined in the issue was also confirmed to work properly.

[alongd]

Good catch! Thanks for the fast fix!
The code looks good, waiting for the tests.

[codecov]

Codecov Report

Merging #1741 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1741      +/-   ##
==========================================
- Coverage   32.61%   32.61%   -0.01%     
==========================================
  Files          87       87              
  Lines       26102    26103       +1     
  Branches     6869     6870       +1     
==========================================
  Hits         8514     8514              
+ Misses      16629    16619      -10     
- Partials      959      970      +11

Impacted Files	Coverage Δ
rmgpy/molecule/molecule.py	0% <0%> (ø)	⬆️
rmgpy/data/statmech.py	42.2% <0%> (ø)	⬆️
rmgpy/rmg/pdep.py	12.21% <0%> (ø)	⬆️
rmgpy/reaction.py	0% <0%> (ø)	⬆️
rmgpy/data/kinetics/family.py	48.28% <0%> (ø)	⬆️
rmgpy/yml.py	15.71% <0%> (ø)	⬆️
rmgpy/data/kinetics/database.py	50.61% <0%> (ø)	⬆️
rmgpy/rmg/input.py	34.34% <0%> (ø)	⬆️
rmgpy/statmech/ndTorsions.py	59.78% <0%> (ø)	⬆️

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[alongd]

Thanks for this fix!