Split PISM ocean models into "sub-shelf-surface" and "ocean proper"

[ckhroulev]

Similarly to surface and atmosphere models for the top boundary, PISM should have shelf base and ocean models (instead of one "ocean" lumping ocean forcing with parameterizations of sub-shelf melt).

Rationale: atmosphere models make it relatively easy to modify inputs of a top-surface-process model (using scalar offsets, for example). Currently ocean modifiers allow adjusting outputs of a selected ocean model, but not its inputs.

We would like to be able to use scalar offsets to modify the potential temperature of seawater used as an input of the -ocean th model, for example.

[tkleiner]

Is this still on the list? I think PISM would benefit.

[ckhroulev]

Yes, I still consider this worthwhile but we don't have the resources to work on this at the moment.

Please let me know if you (or somebody you know) would like to contribute the code. I'd be happy to help with such an effort.