Merge branch 'smoothing_length_derivative' of gitlab.cosma.dur.ac.uk:swift/swiftsim into smoothing_length_derivative

First version of the multiple time-stepping

Eventually...

That should open up debugging from a larger crowd.

See merge request !79

libnuma, immintrin.h detection in configure script

Add a check for the numa.h header (not just the libnuma binary) and a more robust check for vector.h's inclusion of immintrin.h (currently false negative). Also enable example/main.c's -o option - handled, but absent from getopt call.

This is the last of the non-vectorisation changes from local_position_cache.

See merge request !84