Removed unnecessary aspects of StatMech

[nateharms]

After chatting with @davidfarinajr, it was brought to our attention that we were over-specifying information in our Arkane input files. Specifically:

• We didn't need to specify the number of atoms and the atom types
• We didn't need to specify the number of bonds and the bond types
• We didn't need to specify the external symmetry

Arkane will do all of this on its own. This PR removes those specifications in statmech.py and corresponding tests. My one question: would there be a time when we would want to specify these information instead of relying on parameters determined from Arkane?

[codecov[bot]]

Codecov Report

Merging #66 into master will increase coverage by 0.03%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
+ Coverage   63.78%   63.82%   +0.03%     
==========================================
  Files          27       27              
  Lines        4667     4567     -100     
==========================================
- Hits         2977     2915      -62     
+ Misses       1690     1652      -38     

Impacted Files	Coverage Δ
autotst/calculator/statmech.py	70.96% <ø> (+3.44%)	:arrow_up:
autotst/calculator/statmech_test.py	94.59% <ø> (-0.41%)	:arrow_down:

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[nateharms]

Sorry, couldn't merge this at the moment, rebased and force pushed so once the tests pass, I'll merge in. Thanks for the review @davidfarinajr