reimplemented porosity in this sense: now hydrology_englacial_porosity is the 'true' porosity associated to englacial storage and mass conservation, while the sum hydrology_englacial_porosity+hydrology_regularizing_porosity is the diffusive regularization of the pressure equation; merely runs slowly if hydrology_regularizing_porosity=0
Test P clarified; corresponds to zero englacial storage (thus hydrology_englacial_porosity=0) and 'normal' amount of regularizing_porosity; runs fast enough so we can go to 641x641 grid