fix: fix unknowns not present in initialization system in linearization

[AayushSabharwal]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[ChrisRackauckas]

Test the case I described, x => 2, x ~ y, and y => 2, x ~ y.

[AayushSabharwal]

Does this test work? It passed locally, just making sure it's what you're looking for

[ChrisRackauckas]

No it needs to be structurally simplified.

[baggepinnen]

what happens if there is something for which !is_parameter(sys, p)?

[AayushSabharwal]

That's a bug, thanks 😅

[baggepinnen]

Haha, you fixed something other than I had in mind actually, let me rephrase the question after the fix

what happens if there is something for which !is_parameter(sys, k)?

note p -> k

[AayushSabharwal]

Ah. Well that piece of code handles parameters specified as a Dict. Its sole purpose is to update the MTKParameters object. If k is not a parameter, it's not in the parameter object, so it's ignored. If it's an unknown, it's already taken care of in u.

[AayushSabharwal]

No it needs to be structurally simplified.

structural_simplify happens inside linearization_function, in io_preprocessing.