Add li cation

[rmatsum836]

PR Summary:

This PR adds a SMARTS string definition for the lithium cation, opls_406. Looking at the gromacs documentation, I don't see the reference for this atom type: https://github.com/gromacs/gromacs/blob/master/share/top/oplsaa.ff/atomtypes.atp#L425. I'll try to keep looking so I can add a DOI for it.

Test files for the lithium cation have also been added into the opls_validation folder and test_opls.py has been run to add the molecule to the list of implemented OPLS molecules. When I ran this, additional molecules were added to the txt file. Perhaps these molecules weren't added in their respective PRs?

PR Checklist

---

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

[codecov[bot]]

Codecov Report

Merging #383 (4f595e2) into master (2db95ac) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #383   +/-   ##
=======================================
  Coverage   71.28%   71.28%           
=======================================
  Files          16       16           
  Lines        1508     1508           
=======================================
  Hits         1075     1075           
  Misses        433      433