Fixed bug with intrinsic dopair1. Break in search loop and there is no need to pad cache and check for r2>0 in dopair as particles past the threshold will return false in r2<hig2.

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