Large set of improvements to the hydro interactions functions:
- Correct version of DOPAIR2() (Fixes #361 (closed)). 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 This is superseded by @jwillis' work.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
cjfor PAIRs and
cfor 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.