Merge branch 'DOPAIR2_fix' into 'master'
Dopair2 fix Large set of improvements to the hydro interactions functions: - Correct version of DOPAIR2() (Fixes #361). The symmetric condition was not correctly taken into account when computing the distance on the axis. - Removed the old vectorization scheme from the interaction functions. This is superseded by @jwillis' work. - Added the check for active/inactive in the *_NAIVE() functions. These can now correctly be used throughout a run. - Made all the interactions take place in the frame of the cell `cj` for PAIRs and `c` for the SELFs. - Added a new particle type. The particles do nothing apart from recording who they interact with in the density and force loops. This is solely designed for debugging. See merge request !419