feature request: make remake take partial parameter maps

[TS-CUBED]

It would be great if on a model with, let's say, 100s of parameters one could simply change a single parameter without having to create the whole parameter map.

Something like:

remake(prob, p=[p8 => 12])

would change the parameter named p8 and keep all the other parameters the same.

My models come from MTK, acausal modelling, so the parameters are not always in the same order (or the order I define them in), but I also leave many at the component defaults. So I do not really need a full parameter map of all the few hundred parameters, if I only want to optimise for 10 of them.

At the moment I need to create the full parameter map, then use varmap_to_vars() to create the full parameter vector and then remake() to change the parameters in the ODEProblem.

Discussion about this in Discourse: https://discourse.julialang.org/t/is-there-a-way-to-change-a-single-parameter-in-am-odeproblem-and-rerun-it/86775

[ChrisRackauckas]

It does now.