Move `chempot_diagram`, `phase_diagram` and `reaction_calculator` to `pymatgen-core`?

[kavanase]

Unless I am missing something; I would suggest that pymatgen.analysis.phase_diagram, pymatgen.analysis.reaction_calculator and pymatgen.analysis.chempot_diagram be moved to pymatgen-core. I believe these sub-modules are as stable, if not more so, than much of the code that was moved to pymatgen-core, they are core to many materials analyses / pymatgen use cases, and as far as I can tell do not depend on code outside of pymatgen-core.

As a side benefit, these sub-modules are the only modules not in pymatgen-core which are required by e.g. doped, ShakeNBreak, pymatgen-analysis-defects and (I suspect) many more packages. Moving them to pymatgen-core would allow these and many other downstream packages to depend on pymatgen-core rather than the full pymatgen.

[shyuep]

Agreed.