fix(exg): trap multiple connections per cell pair

[wpbonelli]

Warn if any pair of connected cells in a like-model exchange has multiple connections. This will be an error for PRT-PRT exchanges, but multiple connections are tolerated for other models for flexibility.

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Checklist of items for pull request

• Replaced section above with description of pull request
• Added new test or modified an existing test
• Ran ruff on new and modified python scripts in .doc, autotests, doc, distribution, pymake, and utils subdirectories.
• Formatted new and modified Fortran source files with fprettify
• Added doxygen comments to new and modified procedures
• Updated develop.toml with a plain-language description of the bug fix, change, feature; required for changes that may affect users
• Updated input and output guide
• Removed checklist items not relevant to this pull request

[wpbonelli]

Re: deciding whether this should be a general restriction or only apply in some cases, @mjr-deltares figured that people may be using DISU in nonstandard ways involving multiple connections between the same two cells. E.g.

90% of the shared face has a low conductivity and the 10% has a high conductivity to model some kind of geological feature or artifact from mining

[aprovost-usgs]

Re: deciding whether this should be a general restriction or only apply in some cases, @mjr-deltares figured that people may be using DISU in nonstandard ways involving multiple connections between the same two cells. E.g.

90% of the shared face has a low conductivity and the 10% has a high conductivity to model some kind of geological feature or artifact from mining

Guess it comes down to whether the risk of a problem due to accidentally duplicative connections going unnoticed outweighs the benefit for creative modeling like @mjr-deltares described. Maybe consider a warning?

[wpbonelli]

Maybe consider a warning?

Switched to a warning