Improved isotope support in converter and translator modules

[mliu49]

Motivation or Problem

Previously, we couldn't generate identifiers for isotopic molecules because the isotopes would be lost on conversion to an RDKit or OpenBabel molecule.

Description of Changes

Properly transfer isotope information when converting to and from RDKit and OpenBabel molecules. Also add a few unit tests.

Testing

Check that Travis tests pass. Manually test some other molecules of interest.

[codecov]

Codecov Report

Merging #1453 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #1453      +/-   ##
=========================================
- Coverage   40.81%   40.8%   -0.01%     
=========================================
  Files         179     179              
  Lines       30889   30895       +6     
  Branches     6291    6293       +2     
=========================================
  Hits        12608   12608              
- Misses      17399   17405       +6     
  Partials      882     882

Impacted Files	Coverage Δ
rmgpy/molecule/converter.py	0% <0%> (ø)	⬆️

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[goldmanm]

Excellent PR!

It doesn't cover SMILES conversion, but I will tackle this in another PR. Thanks for the excellent improvement.