IcyDwarf v14.6.1

Issue with too much NH3 at the seafloor now seems solved by getting out
of dehydrate() if the routine turns out to hydrate rock.

Next: deal with crack healing at lower temperatures than the current
Tdehydr_max = 850 K. At 850 K, cracks heal in 1 yr rather than 50, the
time step. Keep track of how long cracks have been open and whether they
should heal.