Add initialization message for NullTransport default choice.
In the Mohr-Coulomb model, i.e. the Compressible-Coulomb-Plastic model of Tulaczyk 2000a, made the lower bound on effective pressure N_0 scale with overburden pressure through new configuration constant till_effective_fraction_overburden (=delta in notes).  Removed previous configuration constant till_reference_effective_pressure (=N_0 itself in notes).  New behavior for Greenland rather like Bueler & Brown 2009 result, i.e. in the NullTransport case.  (Which is basically the desired behavior if we are working on better hydrology.)
Allows control of till_effective_fraction_overburden (=delta in notes) for quick parameter study.