Rename configuration parameter till_c_0 to till_cohesion, as clearer and avoiding naming by variable name.
With this modification we have a precise relationship between the N(e) formula in Tulaczyk et al 2000 and the N_til(W_til) formula documented in Bueler and van Pelt (submitted soon).  This changes make no meaningful difference to Greenland tau_c results, as far as I can tell.