fix unreported bug: nc file left open when PISMRoutingHydrology attempted to find bwat in input file
add ability to start -hydrology distributed model if bwp is missing; uses bootstrapping_bwp_value_no_var value
Create new option and config flag -tauc_add_transportable_water.

This option will work in PISMRoutingHydrology and derived (e.g.
PISMDistributedHydrology).  In PISMNullHydrology it is an error.
It changes the computation of tauc from the water amount.  Instead
of just using tillwat, it uses tillwat+bwat.

Testing of this new feature:  It compiles.  Error messages seem to
occur when appropriate.  In all other ways you are on your own.