Merge branch 'dopair-vectorisation-merge' into 'master'

Dopair1 vectorisation merge

Summary
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- Contains a vectorised version of `runner_dopair1_density`
- `runners` now have two particle caches to hold cell ci and cell cj
- The `cache` has been updated to include `max_d` which is the maximum distance into the neighbouring cell
- `cache.h` contains more functions to read two cells into cache and to read a subset of particles from each cell into the cache


See merge request !320