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 This is superseded by @jwillis' work.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