Fix basal B.C. for enthalpy (in 0.5)

Now PISM will always use the insulating Neumann B.C. for enthalpy at the
base *if* the base is temperate. We no longer follow the decision chart
in [Aschwanden et al].

Previously PISM set B.C. in the temperate base case as follows:

- insulating Neumann B.C. if a non-zero-thickness layer of basal
  temperate ice is present, and
- Dirichlet B.C. otherwise

(cherry-picked from 3f30b27)