Patch vectorised pair interactions

Implements small optimisations to `runner_dopair1_density_vec` suggested in #354:
 - Break out of the loop checking for active particles if the cell itself is inactive
 - Do the calculation in the frame of `cj` and not `ci` since this is where the shift vector brings us.
 - Correctly apply the rshift value on the axis linking cells.

See merge request !407