Use better regridding methods

[aekiss]

datm.streams.xml and drof.streams.xml in the ACCESS-OM3 configurations currently use <mapalgo>bilinear</mapalgo> but there are better options - see here and here. Probably best to use conserve for fluxes and patch for everything else - this is what we did in ACCESS-OM2.

See https://github.com/COSIMA/access-om3/wiki/Configurations for details.

[aekiss]

Also see remapMethod in nuopc.runseq; options are redist, bilinear (the default), patch, nearest_stod, nearest_dtos, conserve. For strict bit-for-bit reproducibility srcTermProcessing=1 and termOrder=srcseq are also required. See details here and here.

[kieranricardo]

As far as I can tell, at the moment the field exchange modules in cmeps (e.g. here) assume that redist is used. This way all fields arrive at the mediator on their native grids and CMEPS handles the re-gridding itself.

The regridding options are set by the field exchange modules and we'll need to create our own ACCESS field exchange module so this gives us a bit of flexibility. The existing field exchange modules enable remapping fields to be specified, if these aren't specified the remapping type falls back to a hard coded default and the weights are calculated at initialisation. We could follow this approach or do something else (e.g. allow uses to specify a remapping type).