Toggle num threads before single-physics driver execute

[RonRahaman]

This is a workaround for the fact that nekRS forces the number of OpenMP threads to be 1:

https://github.com/Nek5000/nekRS/blob/d955b9b75cf50ca525dfc2f9c5a38e8c622e5d59/src/core/utils/occaHelpers.cpp#L92-L93

The nekRS devs confirmed that this will be removed in upcoming versions. At that point, we should be able to revert this PR.

In this PR:

• Each single-physics driver discovers the number of OpenMP threads at the end of its constructor. It sets the num threads to a data member, num_threads
• In the coupled driver, the number of threads is toggled directly before each single-physics driver executes.

[RonRahaman]

(edited PR description with more clarity)