Symmetry number calculation capability

[googhgoo]

With the atomic structure of the molecule, we can automatically calculate the symmetry number, which is currently not automated. I'm automatically calculating it using pymatgen and a look up table. It could be a great addition to pMutt to add this functionality. A note is that calculating symmetry is not a trivial task, so it would save a significant time to just use pymatgen function (thus we need to consider adding another dependency). If anybody can give me some direction, I can implement the code.

[jonlym]

That sounds super useful!

I'd suggest adding lines to pmutt.statmech.rot.RigidRotor along the lines of:

if symmetrynumber == 'pymatgen':
    symmetrynumber = get_symmetry_number_from_pymatgen(atoms)

Then add the required version of pymatgen to setup.py.

Then you can submit a pull request to the development branch and we can iterate from there.