Threadpool task plots2

Should fix the problems in the rolled-back merge request of the previous branch, `threadpool_task_plots`.

Which adds the following:

 - --enable-threadpool-debugging to the ./configure flags,
 - -Y to main.c to enable dumping threadpool timers much like the task timers.

This generates threadpool_info-step%d.dat files every -Y steps which contain the mapper function name, the id of the thread that executed it, the number of chunks processed, and the tic and toc timers for the mapper function.

In this merge we also have new scripts to plot and analyse these files and have a new
technique using barriers to start and control the threads. This gives significant
speedups in start up time and time to collective wait (i.e. coming together at task
completion).


See merge request !385

Documentation of the FMM-MM code



See merge request !387

Merge branch 'gravity_documentation' of gitlab.cosma.dur.ac.uk:swift/swiftsim into gravity_documentation

First version of the documentation of the MM scheme. Still need to work out the correct analytical expression of the truncation formula in Fourier space.

Conflicts:
	src/threadpool.c

More conflicts resolved as the merge point seems confused after the
previous revert of threadpool_task_plot branch merge.

Disc patch x

Replaced disc patch potential in z with a disc patch potential in x. Added a 1D version of the disc patch test. Changed the GIZMO time step criterion (fixed a bug in the original criterion, and tried to make the criterion stricter to cope with the disc patch potential). Removed some gravity variables that were no longer used. Put the flux limiter in a separate file and made it configurable.

See merge request !382

Revert "Merge branch 'threadpool_task_plots' into 'master'"

This reverts merge request !375.

There is a problem running the testThreadpool program when built with:
```
./configure --disable-optimization
```
it is crashing during the program exit phase.

See merge request !383

Mpi drifts

Instead of drifting all particles in the super-cell before sending, drift only the sub-cells that are involved in an interaction. For deep hierarchies, this saves a lot of drifting.

This merge request removes two tests for drifted particles when sending and receiving, as not all particles will be drifted and we can't know which ones need to be drifted there and then. Note, however, that undrifted particles will still be caught in the interaction tasks.

See merge request !378

This reverts merge request !375

Threadpool task plots

Adds the following:

 * `--enable-threadpool-debugging` to the `./configure` flags,
 * `-Y` to `main.c` to enable dumping threadpool timers much like the task timers.

This generates `threadpool_info-step%d.dat` files every `-Y` steps which contain the mapper function name, the id of the thread that executed it, the number of chunks processed, and the `tic` and `toc` timers for the mapper function.

See merge request !375

Partially restored old time step criterion. Isolated flux limiter into a separate file and made it configurable. Removed some superfluous gravity variables.

Fix to make test27cells still give 48 neighbours

Small change as test27fix we want to still output the weighted number of neighbours as described by other people.

See merge request !381

Random fixes

We only need `ti_sort` in debug mode.

See merge request !374

Moved around the energy gravity update so that it uses the old momentum. Removed a superfluous time step in mflux.