NUMA-aware affinity for workers

* This is not vectorisation-specific.
* There may be better trade-offs between HT and NUMA.
* Maybe print a warning when we require multiple NUMA nodes.
* Must also detect when Hyper-Threading is not present.
* Probably better as a configure flag, rather than conditional only upon
  the availability of libnuma.
* ~15-40% performance improvement on COSMA.