Automatic @threaded activation when initializing multi-threaded Julia

Hello Trixi team,

As far as I understand, initializing Julia with more than 1 thread automatically uses them inside Trixi loops (correct me If I am wrong). However, I see benefit in having a keyword stored in cache to activate it, similar to the one of OrdinaryDiffEq (thread=OrdinaryDiffEq.True()), when, for instance, preferring to use threads somewhere else.

Also, when experimenting with examples involving threads, I consistently encounter allocations (which are expected) along with a degradation in performance, e.g. #1596.

Should there be interest in having this feature, I could give it a try. Or you could also suggest a simple fix. Thank you.