EOS errors from ideal ions at high density / low temperature

See discussion on c58b3fc7cae532008ef437231033dd86718764b2, which was reverted.

The ideal ion free energy in Skye gives negative entropy at sufficiently high density. This is a problem not just for Skye, but also for ideal, which is meant to be our backstop EOS. Therefore we end up having no EOS coverage at all in the region labeled 'nothing' in this plot:


The proper physical solution is to account for ion degeneracy, but that might require some tabulations of fermi-dirac integrals?

Start with the discussion on commit c58b3fc7cae532008ef437231033dd86718764b2 and inspect the ideal-ion free-energy path, including the reverted change. Reproduce the negative-entropy region at high density and low temperature, then determine the required degeneracy treatment or Fermi–Dirac tabulations. Done should restore EOS coverage where the plot currently shows “nothing”.

Written by the indexing model from the issue text.