Removed unnecessary arguments to calcRemainder() and perform all interactions at the end of the 1st loop for each pi.

Moved entry struct to runner.h for inclusion in runner_doiact_vec.h and added #ifdef to call vectorised version of dopair1.

Replaced all calls to VEC_RECIPROCAL and VEC_RECIPROCAL_SQRT with vec_reciprocal and vec_reciprocal_sqrt.

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.