Expose several additional options in sesame2spiner

[Yurlungur]

It would be useful to expose

• USE_MAXWELL_TABLE to get the maxwell constructed table if available
• EOS_SPLIT_COWAN to turn on Cowan splitting explicitly for electron/ion splitting if needed
• EOS_CALC_FREE_ENERGY to get a free energy if it's unavailable even if it's less good than a first-principles version

[Yurlungur]

In the case of USE_MAXWELL_TABLE, we could also take advantage of our knowledge of a Maxwell construction to floor the pressure.