Modify opls_validation ".top" impropers

[CalCraven]

This PR aims to fix bugs in the opls_validation files. More specifically, improper parameters that were added to those files for testing do not using the correct "funct" value to designate them as periodic impropers. They had previously all been given the value of "1", which designates them as periodic propers. Parmed doesn't treat periodic propers any differently than periodic impropers, except to use a "dihedral.improper" boolean. They are still stored in structure.dihedrals.

This is relevant when doing conversion from a parmed object to GMSO, as GMSO has explicit container for impropers. This will be necessary when parameterizing in GMSO as opposed to ParmEd, and testing that the parameters are stored in this impropers container in GMSO.

See this link for more information about gromacs topology files. https://manual.gromacs.org/current/reference-manual/topologies/topology-file-formats.html

• Change funct values from "1" to "4" in opls_validation top files
• Add test to "test_opls.py" to validate the correct number of impropers are found

PR Summary:

PR Checklist

---

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

[codecov[bot]]

Codecov Report

Merging #506 (d3505d1) into main (24a5d5c) will not change coverage. The diff coverage is n/a.

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