Add getindex/setindex! methods for MTKParameters with ParameterIndex

[MasonProtter]

Without this method, we hit the generic methods for MTKParameters which assumes ParameterIndex is an integer

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

[AayushSabharwal]

pind.idx is not necessarily a 2-tuple. It can also have extra indices in case p.portion[i][j] is an array. For example, for @parameters p[1:3] you might have a ParameterIndex(Tunable(), (2, 1, 2)) to refer to p[2].

[MasonProtter]

I see, thank you. does this change I just pushed make more sense?

[AayushSabharwal]

I forgot this last time around 😅 but I think setindex! should just call set_parameter!. It handles the extra indexes, doesn't allow setting the dependent portion (as intended) and updates the dependent values.

[AayushSabharwal]

Suggested change

	elseif portion === DEPENDENT_PORTION
	setindexer(p.dependent)

The dependent portion shouldn't be set directly

[MasonProtter]

Okay, I'd switched it to just fall back to set_parameter

[AayushSabharwal]

LGTM, but I don't have permissions to make CI run.

@ChrisRackauckas could you please approve CI

[ChrisRackauckas]

Needs formatting

[MasonProtter]

I added a basic testset to try and cover all the branches. Let me know if that's appropriate.

[ChrisRackauckas]

Format's last PR