Extending support for MITgcm configurations

[hdrake]

Phase 1: Extend support for simpler topologies, such as SOSE, a single face of an LLC grid, or idealized configurations

This should only require appropriate specification of the xgcm.Grid and an MITgcm.yaml file.

Phase 2: Extend support to arbitrary grid topologies, such as global LLC grids

This should be fairly more straightforward to implement in the default case of integrate_transport = False in xwmb, since all that is needed is a tracer cell mask. The lateral convergence of mass transports (already transformed into lambda coordinates) will require some interpolation, which will need to make use of face-connections information.

The bigger lift will be for the integrate_transport = True case, in which all of the functionality of sectionate will need to be generalized to account for transport sections that cross faces with arbitrary topologies of xgcm.Grid.face_connections.