No shadowed variables

Some small improvements to the code:

 - Force compiler to report shadowed variables. 
 - Removed all shadowed variables. 
 - Added the function scheduler_activate_send() to simplify the unskipping of MPI sends.

This comes after me spending a day and a half tracking a bug that came from a shadowed variable.

See merge request !421

Do not use eps=0 in the gravity derivative unit test. That leads to FPEs when running without optimizations.

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 This is superseded by @jwillis' work.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

Force compiler to report shadowed variables. Removed all shadowed variables. Added the function scheduler_activate_send() to simplify the unskipping of MPI sends.

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.