Skip some tests if optional deps not installed

[esoteric-ephemera]

• Skip tests which rely on optional modules if those modules are not installed:
  • similarity: needs matminer for feature vector search
  • alloys: needs pymatgen-analysis-alloys
  • MPRester: needs mpcontribs-client for pourbaix and cohesive energy functionality
• Re-enable pourbaix tests

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.44%. Comparing base (d1f199d) to head (96050ec).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1045      +/-   ##
==========================================
+ Coverage   74.04%   76.44%   +2.39%     
==========================================
  Files          50       50              
  Lines        2793     2793              
==========================================
+ Hits         2068     2135      +67     
+ Misses        725      658      -67     

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