Document and Test Dual Sign Convention

[michaelbynum]

Fixes #3519.

Summary/Motivation:

This PR adds documentation and tests for the dual sign convention adopted in the new solver interfaces.

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[bernalde]

I believe this affects MindtPy and GDPOpt as in the implementation of outer-approximation algorithms; we are making assumptions on the directions or the multipliers
@ZedongPeng @emma58

[michaelbynum]

Are MindtPy and/or GDPOpt using the new solver interfaces in contrib/solver/?

[bernalde]

Are MindtPy and/or GDPOpt using the new solver interfaces in contrib/solver/?

Not that I know of, but it might be worth revisiting this

[emma58]

Are MindtPy and/or GDPOpt using the new solver interfaces in contrib/solver/?

Not that I know of, but it might be worth revisiting this

There's nothing to stop someone from using them as subsolvers--we have those as config arguments. And we aren't testing with them currently, so I think it is possible we could be running afoul of the dual sign convention? But of course, it was before too since we let anyone stick any solver in there...