siconca is equivalent to siconc, presents more data opportunities

[kyleredilla]

It looks like siconca is the equivalent of siconc but on the "atmospheric" grid, and it has a substantial representation in the CMIP6 holdings as well, including some that are not available under the more common siconc. We should probably add this to the transfers pipeline. For example, siconca should be globus-ready for CESM2 even though it is not for siconc. But we need a mechanism to join these sets for derived products like regridded data!

edit: we do have CESM2 siconc, but there still could be others found through the siconca variable.