Restore OpenMP support

Thanks to the rework of ndarray, everything is syndicated on a single
pair of functions!

Beware that nested parallelism is disabled, so parallelization of the
iteration over an int[2][1000000] is not good, while parallelization of
the iteration over int[1000000][2] is great. What can we do about this?
Merge pull request #246 from serge-sans-paille/restore-openmp-support

Restore OpenMP support