Replace the bad choice for a W <--> Wtil transfer model with simpler (null) version.

The "transfer model" involving trying to make Wtil evolve toward a fraction
of the W amount has never worked.  So I am going back to something simpler
that will make coupling to ice dynamics much more straightforward.

The replacement model is basically the -hydrology null model.  The rate of
change of Wtil is the melt rate.  Conservation occurs (in -hydrology routing
or -hydrology distributed) by the same method, namely that d(W + Wtil)/dt
evolves by a given formula.

I now assert that implicitness of the d Wtil/dt was never the issue, though
maintenance of bounds  0 <= Wtil <= Wtilmax remains just as important.

Documented these changes in the user manual.

Config parameter (and option) changes:
  hydrology_tillwat_rate                 GONE
  hydrology_tillwat_transfer_proportion  GONE
  hydrology_tillwat_decay_rate_null      RENAMED: hydrology_tillwat_decay_rate
Minimal maintenance on Nbreen example.
tool to generate scatter plot of P versus W, to examine evidence for P(W) a la Flowers and Clarke
Being more careful about meaning of convergence of Test P.
Word "lost" should be "loss".
Clean up scripts.
Merge branch 'dev' into hydro-simplify-transfer
Use CRITICAL_FILL_MISSING flag and fix committed by ckhroulev in 1f14abf.  See 23dda01.  Works for me.