Collapsing combining of Forcefield when loading from multiple files

[daico007]

PR Summary:

Relate to #440 and #441. Enforcing that combined forcefield (loaded from multiple files) must have the same combining rule. Then collapse the combining rule to a single string (fix the bug found in #440). Also, remove the combining_rule option in the apply step.

PR Checklist

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• [x] Includes appropriate unit test(s)
• [x] Appropriate docstring(s) are added/updated
• [x] Code is (approximately) PEP8 compliant
• [x] Issue(s) raised/addressed?

[codecov[bot]]

Codecov Report

Merging #442 (165b6c4) into master (af0cb08) will increase coverage by 0.04%. The diff coverage is 87.50%.

:exclamation: Current head 165b6c4 differs from pull request most recent head 0a18fb5. Consider uploading reports for the commit 0a18fb5 to get more accurate results

@@            Coverage Diff             @@
##           master     #442      +/-   ##
==========================================
+ Coverage   73.84%   73.88%   +0.04%     
==========================================
  Files          17       17              
  Lines        1839     1842       +3     
==========================================
+ Hits         1358     1361       +3     
  Misses        481      481