GridToMeshESMFRegridder: Allow use of mesh objects for target mesh

[arjclark]

✨ Feature Request

It would be incredibly useful when generating regridders for GridToMeshESMFRegridder to be able to use mesh objects rather than requiring a cube for the target.

Motivation

This is motivated by the need to generate ancillary files for running LFRic. In NGMS an LFRic mesh generator is used to provide mesh files for how the model should be set up. These are loadable by iris using the load_mesh routine and all seem correct, etc. However, when it comes to constructing a regridder you can't use them in operations like: rg = GridToMeshESMFRegridder(regridme, targetmesh) as you get the error: AttributeError: 'Mesh' object has no attribute 'mesh'. Thus, in order to use the mesh we would then need to write code to generate an empty cube to attach the mesh to. This is redundant and unnecessary boilerplate.

@stephenworsley

[stephenworsley]

closed by #289