Note that this patch does not introduce any real temperature
dependence but only changes the APIs for the viscosity and for the
density related methods. Note that I also don't like the fact that
this requires so many changes to so many files, but with the current
design of the property classes I cannot see a way to avoid this...
Add new function is added that computes saturation from surface volumes
solving z = As for each cell. This function is used to compute an
intial guess to the saturations in initState_impl.hpp.