Create generic regridder from user provided weights.

[stephenworsley]

✨ Feature Request

Following on from ideas in #4754, design a regridder/scheme which allows arbitrary regridding with user supplied weights. The idea being that a user can calculate weights for unusual types of regridding, put them in a standard form and have Iris handle the rest of the regridding process. This object would be initialised with a source cube, target cube and regridding weights provided in some form (probably as a sparse matrix).

Potential examples of use:

regridder = CustomRegridder(source, target, weights_matrix)
result = regridder(source)

or else:

result = source.regrid(target, CustomScheme(weights_matrix))

There is some potential for also specifying the masked data handling behaviour with something like mdtol in AreaWeighted. There is also some potential for flexibility with the types of coordinates this works over.

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[trexfeathers]

I'm assuming this one needs to stay open @stephenworsley ?