Logger multithreading

Still waiting on !1294 (merged) in order to add the number of threads in the parameter structure.

Nothing fancy here, I am just rewriting the read_all functions in order to use a threadpool. While the code is a bit slower now on 1 thread, I see a speedup of 2.3 when using 4 threads compared to the previous version. The speedup obtained is for the SedovBlast3D, therefore we are still dealing with a relatively small example where setting up the threadpool takes a non negligible amount of time.