Fixed hydrology test script so it has a minimal chance to work with current implementation.
Made subglacial_water_pressure() pure virtual in base class PISMHydrology.  It has the same implementation in PISMNullTransport hydrology for fundamentally different reasons.
Made model in -hydrology null match notes (with additional constant decay rate only).
Fixed up test script so comparing apples to apples in -hydrology null and -hydrology routing.  Runs are longish but easy to interpret.  (To do list is: Fix FIXMEs in PISMRoutingHydrology.)