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.
Fix a stupid bug in the fix for an unreported bug.

(See b102325.)
Merge branch 'dev' into hydro-simplify-transfer
Use CRITICAL_FILL_MISSING flag and fix committed by ckhroulev in 1f14abf.  See 23dda01.  Works for me.
Vastly more powerful scatterplot generator for testing whether points show emergent P(W).  (And Python + Matplotlib rocks.)
Updating hydrology parameters to match paper and its choices.  Removing overrides mechanism (as code duplication) from nbreen example.
Remove an incorrect FIXME.
Remove incorrect mention of Storglaciaren in intro material in manual.
Further improvement of scatter plot generator for P(W) evaluation.
Fix bug in color scale in script.
Moved script showPvsW.py to pism/green-hydro where I am actually using it.
Add option -till_log_factor_transportable_water which sets parameter of same name.
Remove incorrect FIXME in script.
Several references relevant to green-hydro stuff.
Maintenance on references.