Add "easy" GEOS-IT restart remapping

[mathomp4]

@wmputman has said easy remapping from GEOS-IT (like we have from MERRA2) would be useful to have in remap_restarts.py.

Once @wmputman can point us to where the GEOS-IT restarts are, we could then add an option to make it so.

Note: We'd of course need to know what the horz and vert resolutions are, what the ocean used is, and what the land bcs used is. But once we know that, I believe @weiyuan-jiang could quickly incorporate it.