Add pseudothickness to write_model_dataset for Omega

[cbegeman]

Add the pseudo-thickness to model datasets for Omega initial conditions.

Surface pressure (constant value from config file), temperature, salinity and layer thickness are used to compute the pressure at layer interfaces. One iteration is currently used, pending testing of how many iterations is needed for convergence to desired accuracy.

The pressure at interfaces and a reference density (from config file) are then used to compute the pseudo-thickness.

The pseudo-thickness is automatically added to any write_model_dataset call when Omega is the ocean model AND the optional config argument is provided. (Note: checks for presence of config argument are still needed)

Fixes #461

Depends on #440 for routines related to computing p_interface from model dataset.

Depends on E3SM-Project/Omega#327 to rename LayerThickness to PseudoThickness
OR we can map variable name PseudoThickness (added to MPAS-O dataset before variable mapping) to LayerThickness

Checklist

• User's Guide has been updated
• Developer's Guide has been updated
• API documentation in the Developer's Guide (api.md) has any new or modified class, method and/or functions listed
• Documentation has been built locally and changes look as expected
• Testing comment in the PR documents testing used to verify the changes

[cbegeman]

Testing

Only the manufactured_solution/convergence_both/default case has been run on chrys with intel with this feature. I also ran cosine_bell/restart through init to make sure that the absence of config argument caused pass-through.

[xylar]

I think this should be zero