CVMDriftOnline segfaults on MacOS

[ascillitoe]

Since numba 0.56, the CVMDriftOnline detector segfaults on MacOS (during our CI). See #573.

I have done quite a bit of digging and can't find any out-of-bounds issues or other problems. The tests also pass on linux and Windows. The segfault occurs in the numba decorated _ids_to_stats function. It only occurs when parallel=True and the OpenMP threading layer is used.

A workaround, for now, is to use the numba workqueue threading layer on MacOS. A note has been added to the docs in #573.

In the future it would be good to construct an MWE to open a numba issue. Alternatively, we should check if the issue still occurs when numba 0.57 is released.