Add a mechanism to add additional passes to the end of structural simplification before completion.

[BenChung]

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

This is a preliminary idea for how to do it; we might want to have a trait-like system for this that's more powerful?

[AayushSabharwal]

Suggested change

	if newsys isa ODESystem || has_parent(newsys)
	@set! newsys.parent = complete(sys; split, flatten = false)
	end
	for pass in additional_passes
	newsys = pass(newsys)
	end
	for pass in additional_passes
	newsys = pass(newsys)
	end
	if newsys isa ODESystem || has_parent(newsys)
	@set! newsys.parent = complete(sys; split, flatten = false)
	end

This way regardless of the passes, the system has the appropriate parent. Otherwise if the pass doesn't forward the parent to the transformed system, it'll break user workflows.