Compute Species data from elements

[pbauman]

Somewhat related to #247...

Right now, we manually parse the molecular weight for each species from an ASCII data file... This is obviously not scalable or generalizable. Really, we should be computing molecular weight based on the element composition.

The part that's semi-related to #247 is that we also grab the formation enthalpy and few other things from this data which are only used in the StatMechThermodynamics. So we should separate out that parsing.