Symmetric version of DOPAIR2

Here is a much much better version. I now reproduce the pattern that was used in the original version but with correct loop bounds and use of rshift. 

It's now almost on par with the old (but wrong) version. 

See merge request !426

Also remove debugging output

Previously worked the other way around, which is incorrect.

When given a range of 0 autorange the tasks and plot the threadpoool using the autoranged limits of the tasks

Applied @jwillis' correction to the second loop of DOPAIR2. dx_max was entering the external loop with the wrong sign.

Keep maintenance down.

Don't touch the pinning of the main thread when not using affinity.

There is a problem when the cpu mask is empty and using multiple MPI ranks on a node, in that case all the main threads end up pinned to one core during the reading of the initial conditions (and any other tasks done before engine_init is called).


See merge request !423

Modify the inner-loop exit condition to incorporate the distance on the axis calculation rather than pre-compute it.

There is a problem when the cpu mask is empty and using multiple MPI ranks on a node, in that case all the main threads end up pinned to one core during the reading of the initial condition (and any other tasks done before engine_init is called).

In DOPAIR2(), once we have a pj candidate, check the distance on the axis using hj instead of relying only on the earlier hj_max check.

Reverse the loop to find the exit condition in DOPAIR2() to go from the centre to the outside

Follows @jwillis' suggestion in #362.

Also implements an early exit in the outer loop. If no particle can be in range any more (based on hi_max and hj_max), no need to keep checking the next particles.

See merge request !422

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

Reverse the loop to find the exit condition in DOPAIR2() to go from the centre to the outside. Follows @jwillis' suggestion in #362

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