Allow separate time-stepping or "subcycling" into automated mesh refinement updates

As we all know, automated mesh refinement(AMR) allows a pde solver to incorporate information from finer scale processes into the computation of more coarse scale processes. This has the benefit of improving model accuracy and stability in some cases, as well as accelerating compute time in other cases.

Once key issue that accompanies AMR is "subcycling" or local time-stepping, where the solver allows for independent time-stepping of the finer scale process before incorporating those results or fluxes into the coarser scale model. Subcycling can also allow for faster solving of some multi-scale problems, as well as improved accuracy.

After talking to @jlchan , he indicated that Trixi does not currently support subcycling. However, he suggested that I open an issue so that perhaps the group can discuss incorporating this functionality.

The issue names no files, tests, or entry points. Start by reviewing Trixi.jl's existing automated mesh refinement and time-stepping behavior, then determine the design needed for independent fine-scale steps and incorporation into the coarse-scale model; done would mean an agreed subcycling implementation and validation.

Written by the indexing model from the issue text.