Space rebuild

Implements #243. space_split with bucketsort instead of quicksort. Reduce the amount of swaps needed to sort the particles into the correct progeny, should be faster and scale better.

See merge request !284

Gravity only tests

Added more in depth timers for interaction functions and added macros to choose which versioin of DOSELF1 to use.

Changed scenario to a test particle at the center of a unit sphere. The serial results are now compared against the vectorised results using the compare functions and differences are flagged.

Added functions to compare particles' properties and return an error if they differ more than the provided accuracy.

Replaced all vector reciprocals with macros and fixed a bug in the non-sym and sym vector force interactions. Missing grad-h multiply in the sph-term.

Added more vector intrinsics for AVX512 and support for left-packing vectors in multiple instruction sets.