v11: Ask GEOS-IT question first, look for MERRA-2

[mathomp4]

This is the v11 equivalent of #173

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I've received a couple of pings on teams as people try to remap from MERRA-2 which isn't possible at the moment due to the removal of the /archive link at NCCS.

Eventually, some ability to remap from MERRA-2 might be restored, but until then, we should do a couple things:

1. Ask if a user wants to remap from GEOS-IT (as, in truth, this is probably a better starting point anyway)
2. Then ask about MERRA-2, but also look to see if the place where MERRA-2 restarts live is actually there. If not, just move on
3. Abstract out the MERRA-2 and GEOS-IT locations into a constant. That way if we need to duplicate things, it's in one place

My tests show this does work and is zero-diff when remapping from GEOS-IT.