Add GenTS as submodule

[mnlevy1981]

We want to move from our NCO-based time-series generation to the GenTS package

Description of changes:

• Add GenTS as a submodule (and to (cupid-analysis))
• Update CESM workflow to include a flag to run GenTS instead of the current NCO-based time series

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• Have you followed the guidelines in our Contributor's Guide?
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