De-optimizing computation of tauc in preparation for implementing non-pointwise model: replace redundant computation on ghosted points with communication of ghosts.
Added option -tauc_slippery_grounding_lines and config flag tauc_slippery_grounding_lines.  By default it is 'no' PISM-wide but 'yes' in searise-greenland.  Seems to have very desirable effect on the flow near grounding lines because the at-grounding-line thermodynamics that causes tillwat=0 and high tauc is bypassed.  Reduces peak diffusivity if that occurs at these grounding lines, which seems common.