Initial UGRID support

[alioacar]

Hi everyone,

xESMF does not yet have a support for regridding from unstructured meshes to structured grids. I have an initial working path for this and will eventually cover both #115 and #367. uxarray provides a good normalization for UGRID conventions(#414) but adding uxarray as a runtime dependency will expand the maintenance and I think it's not efficient since xESMF is maintained by a relatively small group of volunteers (many thanks to all for your efforts!). The goal is to support generic UGRID-like xarray datasets so uxarray can provide compatible datasets without becoming a required dependency of xESMF.

So the current prototype supports:

• face-centered source data on unstructured meshes
• mesh construction from node lon/lat coordinates and face-node connectivity
• mesh-to-grid regridding
• no node-centered or edge-centered field support yet

Here is the first example, face-centered cubed-sphere mesh input regridded onto a structured target grid:

What still needs to be added for a minimum support:

• more robust UGRID metadata discovery
• support for datasets that provide xyz coordinates directly
• optional derivation of face centers when face lon/lat are missing
• normalization of mesh metadata across model conventions
• clearer handling of different mesh data locations (face/node/edge)

This is an important use case for models such as FESOM, MPAS, and ICON. Once I reach a stage where these models are properly supported, I plan to open a PR.