WIP: refactor: use ImplicitDiscreteSystem to implement affects in callback systems

[vyudu]

Close #2639 #2612

[isaacsas]

@vyudu is this now being called on all affects coming from a JumpSystem? i.e. not just affects in callbacks but affects in jumps themselves? Because reset_aggregated_jumps! has very significant performance implications, and should not be needed for affects coming from within jumps. i.e. it currently isn't used when compiling affects within jumps here,

ModelingToolkit.jl/src/systems/jumps/jumpsystem.jl

Lines 291 to 300 in 1c00326

	function generate_affect_function(js::JumpSystem, affect, outputidxs)
	consts = collect_constants(affect)
	if !isempty(consts) # The SymbolicUtils._build_function method of this case doesn't support postprocess_fbody
	csubs = Dict(c => getdefault(c) for c in consts)
	affect = substitute(affect, csubs)
	end
	compile_affect(
	affect, nothing, js, unknowns(js), parameters(js); outputidxs = outputidxs,
	expression = Val{true}, checkvars = false)
	end

it is only used for affects within callbacks:

ModelingToolkit.jl/src/systems/jumps/jumpsystem.jl

Lines 582 to 583 in 1c00326

	cbs = process_events(js; callback, eval_expression, eval_module,
	postprocess_affect_expr! = _reset_aggregator!)

[isaacsas]

(This is a big PR, so I haven't read enough to understand where this is being used -- I just wanted to double check because of the potential performance implications of calling reset_aggregated_jumps!.)

[vyudu]

Ah I wasn't aware of this distinction, I'll make sure it only gets called inside the callback affects. The JumpSystem stuff is very WIP still since I want to add an optimization that generates the original explicit updates for systems without algebraic equations. My goal is for the codegen for those cases to be the same as it was before.

[vyudu]

Do JumpSystems support functional affects in callbacks?

[isaacsas]

I think they should for callbacks, but the user would need to manually add reset_aggregated_jumps! into their affect function then. I'm not sure if they would be work for affects within jumps though (where there is no need for reset_aggregated_jumps!).

[vyudu]

What are assemble_vrj_expr and assemble_crj_expr for? In general how important is it to keep the ability to generate expressions for the affect functions?