Enhance Restarting Capabilities

[DanielDoehring]

Two features I often miss in restarting is

• Restarting from different (lower) polynomial degrees. Use case would be a cheaper simulation to get somewhat physical fields and then enhance quality. Another use case would be an easier comparison of solutions with different polynomial degrees.
• Restarting a hyperbolic-parabolic simulation from a (purely) hyperbolic simulation, i.e., computing the gradients from the solution. Certainly, in that case the solution does not correspond to the equations at hand, but might still be helpful.

[ranocha]

Sounds reasonable 👍

[sloede]

This would be a great project also for a research assistant or for a PhD student to cut their teeth on working with Trixi.jl.