Add PyVOL plugin

[JarrettSJohnson]

Creating a tracking ticket to add the PyVOL plugin and subsequently any of the issues and PRs on the corresponding github repo. Seems to be commonly used but not maintained any longer?

Github Repo: https://github.com/schlessinger-lab/pyvol
License: MIT: https://github.com/schlessinger-lab/pyvol/blob/master/LICENSE.txt
Documentation: https://schlessinger-lab.github.io/pyvol/index.html

The msms dependency is available in conda-forge in addition to bioconda; however the recipe seems to install in the incorrect path on Windows which doesn't allow PyMOL to pick this up properly: https://github.com/conda-forge/msms-feedstock/blob/219b67c4c4bda4eee7740bca93162f79623ecf26/recipe/bld.bat#L1

(should be %LIBRARY_BIN% rather than %PREFIX%/bin)

I also haven't looked into it too much, but I can assume PyVOL use another surface generator rather than msms?