Add Smarts strings to atom type Methyl-Nitro Aniline

[Argon1999]

PR Summary:

Added Smarts strings to atom type Methyl-Nitroaniline. But could also atom type other atoms.
Added gro, top and mol2 files for Methyl-Nitroaniline, and updated the implemented_opls_tests.txt file with the new molecule.

PR Checklist

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

[codecov]

Codecov Report

Merging #319 into master will not change coverage.
The diff coverage is 94.5%.

@@           Coverage Diff           @@
##           master     #319   +/-   ##
=======================================
  Coverage   84.78%   84.78%           
=======================================
  Files          14       14           
  Lines        1275     1275           
=======================================
  Hits         1081     1081           
  Misses        194      194

[mattwthompson]

Getting more complicated; I wonder if this increases our coverage of any drug molecules.