Clobber a compiler warning.
Fix a bug in ssa_test_cfbc.cc (in 0.5).
Use non-zero initial guess in SSAFD (in 0.5).
Implement the melange back pressure parameterization (in 0.5).

This was requested by Ioana Muresan.

To use, set "-ssafd_mbp" and "-ssafd_mbp_file foo.nc". Here "foo.nc"
should contain the "melange_back_pressure_fraction" variable, ranging
from 0 to 1. 0 means no melange back pressure, 1 -- maximum back pressure.
Values outside this range are allowed and have expected effects,
although their use would be harder to justify.

Also: fix a bug in the computation of the driving stress at the calving
front.