Make CAM output UGRID Compatible: Workflows for Unstructured Grids

Projet Raijin is developing a python xarray extension called 'uxarray' (https://github.com/UXARRAY/uxarray ). This works on the UGRID standard for meta-data for unstructured grids (https://ugrid-conventions.github.io/ugrid-conventions/).

It would facilitate the development of uxarray and improve our ability to use other common tools if we can get CAM (SE and MPAS) output compliant with the UGRID specifications.

I will also open a UXARRAY issue: https://github.com/UXARRAY/uxarray/issues/11

Initial discussion from @gold2718

> Are you talking about adding full grid topology to every history file or the production of separate UGRID files for each SE and MPAS grid (referenced in each history file as a global attribute). We could save both time and space if we collected UGRID files and used existing ones as an input to CAM (for documenting the history files).
>New UGRID files could be created either by the model or perhaps by Patrick's mesh tool (which might be a lot less total work).
>I suppose one advantage of having the history file be an official UGRID file is the ability to document (via the 'location' attribute) whether an MPAS variable is defined on a node or on the face. Of course, there is nothing stopping the uxarray project from following a global attribute pointing to the required UGRID file and using that.

From @erogluorhan :
>Paul Ullrich was wondering whether the fact that ESMF can support UGRID conventions will make it pretty easy to have CAM grids UGRID compliant (https://earthsystemmodeling.org/docs/release/ESMF_8_0_1/ESMF_refdoc/node3.html#SECTION03028400000000000000). He also mentioned that he’ll look into making TempestRemap provide UGRID-compliant files.