Use reference solids to construct the ion reference phase diagram

[mkhorton]

Addresses issue described here: https://matsci.org/t/issue-querying-pourbaix-entries-using-the-mp-api-rester/40651/6

And also closes bug report action item for Fe-C Pourbaix diagram on web app.

[codecov-commenter]

Codecov Report

Merging #533 (df60edf) into main (7ce7c4c) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #533   +/-   ##
=======================================
  Coverage   92.58%   92.58%           
=======================================
  Files          34       34           
  Lines        1309     1309           
=======================================
  Hits         1212     1212           
  Misses         97       97           

Impacted Files	Coverage Δ
src/mp_api/client.py	92.01% <100.00%> (ø)

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