[WIP] Heisenberg

[ncfrey]

Summary

@mkhorton

New functionality to take low energy magnetic orderings, map them to a Heisenberg model, compute exchange parameters, and perform Monte Carlo with the VAMPIRE package to calculate critical temperatures.

• Heisenberg model mapping of magnetic materials for calculating exchange parameters.
• Wrapper for VAMPIRE atomistic simulations of magnetic materials package.

Additional dependencies introduced (if any)

• Vampire5 command line tool for atomistic simulations of magnetic materials.

TODO (if any)

• Added sophistication to screening/sanitization of input low energy ordering data.
• Cleanup VAMPIRE files after execution.

[mkhorton]

Thanks @ncfrey !

[shyuep]

I propose that you name the pymatgen wrappers around vampire "lestat". :-)

[mkhorton]

Looks pretty good, some tests need to be updated (e.g. see test_round_magmoms in CollinearMagneticStructureAnalyzerTest), a few minor PEP8 violations (mostly line length), HeisenbergScreener class definition has extraneous parentheses etc. I haven't had a look at test coverage yet. Overall seems v high quality though + almost ready to merge. Thanks for the good code comments, made reviewing it easy!

[ncfrey]

Thanks for the review Matt! Working on those fixes now.

[mkhorton]

Find it really weird that ReactionDiagramTest failed here ... there's nothing in this PR that should affect that.