Mesh to mesh regridding

[hdyson]

✨ Feature Request

Currently, iris-esmf-regrid supports regridding from a grid to a mesh, or from a mesh to a grid. It would be useful to also have the capability to regrid from one mesh to another mesh.

Motivation

It is likely that the processing pipeline for UGrid orography ancillaries will need the capability to regrid from one mesh resolution to another. Current wavedrag processing for regular grid ancillaries relies on regridding to an intermediate resolution: it is assumed that the UGrid equivalent will be a regrid from the source grid to an intermediate mesh, and then from that intermediate mesh to a model resolution mesh. Note though that the science requirements here are yet to be confirmed.

In addition, we often regrid model output for the creation of new ancillary files for the model. As we move over to the LFRic model, this model output will be on meshes rather than grids.

I'm happy to refine this at a later date when we do have more specific requirements, if that would be helpful.

[stephenworsley]

Closed by #311.