mct_mod to be removed from CESM3

[jedwards4b]

https://github.com/ESCOMP/SimpleLand/blob/cesm2_3_slim/src/utils/spmdGathScatMod.F90#L16

Use of mct_mod is deprecated this code needs to be modified prior to the cesm3 release.
Please provide a timeline for removing this code from slim.

[ekluzek]

This is wrapped up in the work on #75 and #52 on the move to NUOPC. This work has been on hold, so that I can work on dust in CTSM. But, that work has also been held up based on other things that have come up. The larger scientific priority for CESM3 is dust, so that will have to come first. But, yes we want to get SLIM working with NUOPC for the CESM3 release.

In terms of timelines we'll be talking about this as a priority in the new year to figure out when we'll be able to get to it.

I don't actually expect the work here to be that much (although more than something simple and trivial) so when I get to it, the hope is we'll be able to do it quickly.

This also relates to #90 and #84

[ekluzek]

This now planned to happen with the cesm2_3_beta18 planned tag. So SLIM will be removed from CESM tags at that point until work on this can be completed.