Expand and generalise thermo tables

[nichollsh]

Currently:

• Heat capacities are either evaluated as constants, or through the Shomate equation (NIST coeffs.)
• Latent heats are hard coded.
• Mean molecular weight is hard coded.

This is not very extensible to a large range of molecules (i.e. from FastChem, atmodeller). Instead, the model should use lookup tables (which are fast) to calculate the heat capacity as a function of temperature. MMWs can be approximated as the sum of atomic masses (binding energy only makes a small difference). Latent heats will be more complex (Issue #11).

https://janaf.nist.gov/

[nichollsh]

Also need data on critical temperatures and triple points.