Close #99.

Now enthalpy and temperature time-stepping code computes the *grounded*
basal melt rate, which is combined with the shelf base mass flux
provided by an ocean model in IceModel::combine_basal_melt_rate()
(possibly using the fractional floatation mask computed using a sub-grid
parameterization of the grounding line position).

Oce IceModel::combine_basal_melt_rate() is called, the rest of PISM can
use IceModel::basal_melt_rate in both grounded and floating areas.