migrate to pymatgen-core

[rkingsbury]

Summary

This PR replaces pymatgen with pymtagen-core to reduce dependencies and possibly achieve some performance gains, due to recent changes in pymatgen-core.

EDIT: surprisingly, switching to pymatgen-core does not appear to have changed the required dependencies at all. Nevertheless, it should be a smaller, more focused package so the change is still beneficial.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.21%. Comparing base (ec95fb4) to head (b0ee129).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #403   +/-   ##
=======================================
  Coverage   86.21%   86.21%           
=======================================
  Files          14       14           
  Lines        1842     1842           
  Branches      318      318           
=======================================
  Hits         1588     1588           
  Misses        209      209           
  Partials       45       45           

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