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 This is superseded by @jwillis' work.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

Reinstate missing pointer indirection for external gravity dependencies that had disappeared in merge.

Gravity speedup



See merge request !420

DOSELF_SUBSET, DOPAIR1_NAIVE and DOPAIR2_NAIVE should do their calculation in the frame of the cell.