Create long-standing development "GW" branch for GroundWater component development

[ekluzek]

We will create a long-standing branch for development of a GroundWater component that will be beneath MOSART but as a separated component.

I propose we name the branch "gw" and use the CTSM branch naming convention of:

branch_tags/NAME.nXX_BASELINETAG

So for example, you'll have (expanding the branch versions to 3-digits)

branch_tags/gw.n001_mosart1_0_50

@swensosc @slevis-lmwg

[ekluzek]

This work will start as an evaluation of it's usefulness. Long term we wouldn't want it to be locked into being a part of MOSART. A potential option is to add it as a component of CMEPS. Another option would be for it treated as a subcomponent of any river-model, in such a way that it could be coupled into another river model (i.e mizuRoute). In either of these cases the GW component would be spun off into it's own model and added as an external to river models that can utilize it.

As such as the top level infrastructure is developed for the GW component we should work on designing it in a way that it can be spunoff into it's own component.

[ekluzek]

This will be done after #76 comes in, and that tag will be the starting basis for this branch work.