add initial pass at arg_constraints and arg validation.

[erip]

Fixes #120.

[erip]

I'm a bit sketchy on constraints for some of the parameters which get overloaded as tuples (i.e., log_potentials in SentCFG), so review on this seems particularly relevant. Everything else is pretty vanilla.

[srush]

SentCFG is a tuple of reals (triple). Does that work with constraints?

[erip]

I guess that's the question. It seems like typically the expectation is parameters are not composite so you can have a clean mapping of parameter -> constraint. Since log_potentials is still technically "a real" (just in higher dim space) it should be ok... I can look into adding a test to see if we can validate.

[erip]

As an aside, is there a reason CI didn't run on this PR? The GH Actions config looks OK so I'm not sure why I can't see the results of the tests.

[srush]

Kicked off the CI.