Re-factor enthalpy code.
Remove varenthSystemCtx.
Rename vbmr -> basal_melt_rate.
Merge branch 'enthalpy-refactor' into dev

This cleans up the enthalpy code (a little bit).
merge c42b01a341e into till-hydro-dynamics; fix error in till-hydro-dynamics so that now it is till_water_thickness() and not basal_water_thickness() that is read in the enthalpy code for the water at the base