ENH,RF: Use new features in `utils/threading.h` and `newimage::volume` to parallelise voxel-wise operations
- Don't pass a value to
Utilities::NoOfThreads
- this allows it to take a value from theFSL_NUM_THREADS
environemnt variable (or default to single-threaded if that variable is unset) - Use
Utilities::parfor
to parallellise voxel-wise operations
This MR depends on utils!15 (merged) and newimage!40 (merged)