Warn on --omp-nthreads > 8

[effigies]

What would you like to see added in fMRIPrep?

This is often a sign that someone has lots of cores and wants to use them, not that they have come to a considered decision that more than 8 cores is actually helpful for their workload.

Do you have any interest in helping implement the feature?

Yes

Additional information / screenshots

No response