Ability to pass keywords to ESMF, including pole_method

[stephenworsley]

✨ Feature Request

The regrid call in esmpy includes many keywords which aren't yet controlable via iris-esmf-regrid. There may be a case for either adding some of these as keywords to iris-esmfiregrid classes or else the ability to add a dictionary of keywords to pass down to esmpy (when it is suitable to do so).

[stephenworsley]

One underlying issue may be solved by #378.

[hdyson]

One of the keywords for ESMPy that I'd really like to get at is the UnmappedAction - this would help us enormously with debugging. Specifically, rather than IGNORE, we want unmapped cells to result in an ERROR.

[bjlittle]

Reference #391