Add ParmEd to ChimeraX's standard bundles

[Tristan Croll]

I think I brought this up once before, but can't find an existing ticket on the specific question. ParmEd is a library developed in collaboration with AMBER for conversion of parameters between the different force field formats. It's on GitHub at ​https://github.com/ParmEd/ParmEd and the PyPI as a source distribution at ​https://pypi.org/project/ParmEd/. Having it available in ChimeraX would make it trivially easy to give users support for most ligands in ISOLDE: Call phenix.elbow as a subprocess to generate the AMBER parameters, then use ParmEd to convert to OpenMM's ffXML (I already have the method for that in place). The fact that ParmEd is a source distribution makes it hard to ask users to install it for themselves (easy enough in Linux, but Mac and Windows users would have to install XCode or Visual Studio respectively) - but it installs quite trivially once a compiler is present:

/path/to/chimerax/bin/python3.7 -m pip install versioneer ParmEd

The current alternative would be for me to wrap the necessary code in a try...except and print instructions on how to install the prerequisites to the log (or better: "ParmEd is not installed. Would you like to install it now?" once I figure out how to make that work).