Generalise interior EOS

[timlichtenberg]

• Long-term plan: generalise melt and solid phase EOS toward arbitrary refractory Fe/Si/Mg and H/N/C/S mixing ratios
• Related: https://gist.github.com/djbower/032e2154c471ee79ed7db484026d45f4

[djbower]

On this note, there are several options. You can produce lots of lookup data. Or you can produce lookup data that is anchored to a reference state (some fixed mixing ratios, some fixed Fe content, no water etc.) and then perturb this reference state according to analytical functions that capture perturbations in water content Fe, Mg, Si etc. So this is sort of a hybrid approach, where you can hopefully utilise the speed and convenience of analytical functions whilst also capturing the more complex topology of EOS data in lookup files.