Generalize calculation of variable dry air properties for WACCM-X.

An initial step was taken in PR #153, but that implementation is specific to the current list of dry air species used in WACCM-X, i.e., 'O','O2','H', and 'N2'. Adding new species to this list requires code modification in the physconst module. Ideally the properties of the pure species would be obtained directly from the constituents module.

Also in physconst the properties of the fixed troposphere composition of dry air are hardcoded. It would be good to treat this in a generalized way as well.