Add MITgcm kernel

[rabernat]

I also have one of these written up somewhere.

Reference in MITgcm docs: https://mitgcm.readthedocs.io/en/latest/algorithm/algorithm.html#lateral-dissipation

[rabernat]

If we want gcm-filters to work with MITgcm global LLC grids, we will need to implement the exchanges for the LLC grid faces (for both scalars and vectors). We know how to do this, but it's still a pain in the butt to code up. Xgcm knows how to do it but we can't directly use xgcm here because we need a lower-level implementation that works with GPUs.

This kernel will be somewhat different from the others because it needs a third dimension: the "face index". There are 13 square faces for the LLC grids.

cc @cspencerjones, @TomNicholas