Read SMIRFF-enclosed FFs

[j-wags]

Summary

In testing the ability to read all files from a release-candidate smirnoff99Frosst package, I found that ones with an enclosing XML SMIRFF tag (instead of SMIRNOFF) were unable to be read. This PR fixes that.

• Add tests
• Update changelog

[codecov-io]

Codecov Report

Merging #371 into master will increase coverage by 0.04%.
The diff coverage is 90.9%.

@@            Coverage Diff             @@
##           master     #371      +/-   ##
==========================================
+ Coverage   76.93%   76.97%   +0.04%     
==========================================
  Files          19       19              
  Lines        5414     5416       +2     
==========================================
+ Hits         4165     4169       +4     
+ Misses       1249     1247       -2

Impacted Files	Coverage Δ
...enforcefield/typing/engines/smirnoff/forcefield.py	75.36% <90.9%> (+0.14%)	⬆️
openforcefield/utils/utils.py	84.66% <0%> (+0.66%)	⬆️

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[andrrizzi]

Looks good, thank you! I've noticed a possible bug, but I might be missing something because your new test is passing. Feel free to merge when ready.

The only main suggestion I have is to update data/test_forcefields/README.md to briefly describe the two new force field files. I think it'll help to keep track of the differences between the different files, which will probably become difficult to do without some notes in there.

[davidlmobley]

@j-wags Are you sure we don't want to just get those tags converted? Long long ago we called them "SMIRFF" forcefields rather than "SMIRNOFF", but finally decided to change the name. However, the intention wasn't to maintain backwards compatibility, just to switch.

[j-wags]

@davidlmobley

My goal here is to have every FF from the smirnoff99Frosst repo loadable, and all of them up to the 1.0.4(?) release are enclosed with a SMIRFF tag. Do you foresee any danger in loading those? I'm under the impression that the format in those files is identical to the "official" 0.1 SMIRNOFF spec.

[j-wags]

@davidlmobley To clarify -- I want the toolkit to read SMIRFF-enclosed files because the alternatives are:

1. modifying the released smirnoff99Frosst files, which we probably agree is bad, or
2. not being able to read all released smirnoff99Frosst files using the current toolkit. I wasn't under the impression that there was much risk in auto-upgrading the spec of these old files, but if you suspect that there is, we could remove (or do a deep dive into) this functionality.