Re-implement and improve the three-equation model.

Please see src/coupler/ocean/POGivenTH.* and
src/coupler/ocean/holland_jenkins/*.

(POGivenTH should be renamed.)
Add a missing configuration parameter.
Finishing the "three-equation model" code.

This commit adds the "-clip_shelf_base_salinity" command-line option (so
that one can do "-no_clip_shelf_base_salinity") and the corresponding
configuration flag ocean_three_equation_model_clip_salinity, plus
updates documentation in forcing.tex.

This code should be ready to be ported to 0.6.
Merge branch 'ocean' into dev

This adds the new "3-equation" model of the sub-shelf heat and salt flux
balances to the "dev" branch.