MNT: Call `omp_set_num_threads` in addition to `openblas_set_num_threads`

BLAS threading is disabled by default in FSL tools, as it usually does not give any performance benefit. When using an OpenMP build of OpenBLAS, the openblas_set_num_threads function used to call omp_set_num_threads. But this has changed at some point, and now one must explicitly call omp_set_num_threads in order to control the number of threads used by OpenBLAS.