introduce cache array to make discrete-time state update atomic

[baggepinnen]

One step towards solving #2701

system in issue above now produces the correct answer

[baggepinnen]

@AayushSabharwal the test failure indicates that the indices used to index into p are of some magical type? Tests pass locally so I assume this is some recent change upstream?

[AayushSabharwal]

So view won't work either. There are a bunch of branches based on whether has_index_cache(sys) && get_index_cache(sys) !== nothing (which is basically a check for if the parameter object is an array or MTKParameters). The view version is never used for MTKParameters.

[baggepinnen]

There are likely additional compiler-related problems, outlined in https://github.com/SciML/ModelingToolkit.jl/issues/2701