Dopair1 vectorisation merge
Summary
- Contains a vectorised version of 
runner_dopair1_density - 
runnersnow have two particle caches to hold cell ci and cell cj - The 
cachehas been updated to includemax_dwhich is the maximum distance into the neighbouring cell - 
cache.hcontains more functions to read two cells into cache and to read a subset of particles from each cell into the cache