Enable enthalpy and enthalpy defect initialization

[michael-a-hansen]

Specifying the initial condition of a flamelet through the enthalpy instead of the temperature would be nice. Specifying the enthalpy defect also would be nice (using the linear enthalpy as a reference). This would simplify nonadiabatic, nonstrained library creation.

In general we could consider letting a user define a function that parameterizes a Cantera Solution/Quantity given the mixture fraction.