Very minor code cleanup.
Implement the melange back pressure parameterization.

- Implement PISMOceanModel::melange_back_pressure_fraction().
- Use melange_back_pressure in SSAFD.
- Implement SSA_beta, the "beta" drag coefficient (2D diagnostic)
  $\tau_b = \beta \vec u$

The stress balance code needs a better interface: right not some inputs
are passed in through PISMVars, some through method arguments. I need to
clean this up.
Implement -ocean ...,delta_MBP

This is the "melange back pressure fraction" offsets.
Document melange back pressure fraction forcing.