OMP changes

The OMP changes I made are here; pretty minimal.  OMP by default looks at OMP_NUM_THREADS for the number of threads it uses, so getting rid of the explicit omp_set_num_threads and using #pragma omp parallel for in place of #pragma omp parallel seems to do the trick.

What I wrote _doesn't_ seem to work if OMP_NUM_THREADS is greater than N_samplers, which I don't understand.  But in any case, for the simple situation of wanting it to run on only one core, and for your default situation of OMP_NUM_THREADS == N_samplers, behavior seems unchanged.