General parameter definitions in MPL over all processes
For some processes it is possible to define the coefficients in the mass balance equations via the pressure and temperature dependence of the density in the input script (e.g., as a linear relationship), while other processes only accept those parameters as "storage" and "thermal_expansion" terms. There might be other examples too. I think we should strive here for a more general solution for all processes. Otherwise people end up in defining parameters twice or there are baffled by the fact that defining things doesn't necessarily affect things.