global-ref and foreign only support intent `in` or `out`

[ptomasula]

Description

global-ref and foreign variables only allow for intent in or out, but do not permit a mechanism to pass data between processes in a circular manner.

In theory you could set up a foreign variables in Riverine that references TSM's temperature and vice-versa, but doing so in practice results in a circular reference due to the other_process_cls argument needing to be the class definition of the process that defines the variable.

Impact

I'm not certain there is a way to provide updated water temperature data back to Riverine process. This is sort of a hard requirement for any sort of coupling of the Riverine (transport) and TSM processes.

[aufdenkampe]

@ptomasula, here's a summary of our discussion from his afternoon.

Indeed, getting this issue resolved is a requirement for our continued work with xarray-simlab given that every reactive transport modeling effort that we have in front of us requires at least two processes to modify the same variable in turn (i.e. the transport process performs advection, diffusion, and mixing on an aqueous species then the reaction process performs the reaction on the same aqueous species).

This issue was identified and discussed in detail in xarray-contrib#164. It's a long discussion thread, and @benbovy does an excellent job summarizing the problem and solution in:

• Explicit cyclic foreign vars  xarray-contrib/xarray-simlab#164 (comment)

@feefladder got interested in the problem and began developing a solution in his fork's cyclic branch, leading to this PR:

• cyclic foreign vars xarray-contrib/xarray-simlab#177

@benbovy suggested breaking down the complex PR into a series of smaller, more digestible PRs, which @feefladder did over the following days:

• Custom dependencies xarray-contrib/xarray-simlab#178
• Strict check xarray-contrib/xarray-simlab#179
• Docs xarray-contrib/xarray-simlab#182 (with additions to whats-new.rst for all changes)
• update_processes api for checking and custom dependencies xarray-contrib/xarray-simlab#183
• visualize feedbacks xarray-contrib/xarray-simlab#185
• visualize: show variable stages xarray-contrib/xarray-simlab#186

Perhaps we should pull each of the branches from these open PRs into our fork?