Correctly count the number of cell->task links required and only create the necessary number given the engine policy.

Analyse script shows updates and sid

The task plotting scripts have been updated as follows:
 - Output the number of active particles
 - Output the time spent in pair/sub-pair tasks split by sid
 - Implemented @d74ksy's suggestion to put the legend outside the plot area itself.

See merge request !360

Applied @d74sky's suggestion to put the legend of the plots outside of the plotting area to avoid overlap

The task plot analysis scripts now also displays the number of particle updates done in that time-step. It also shows the time distribution for pairs sorted by SID.

plot-analysis: updates to breakdown deadtimes

updates to the plot analysis scripts so that the deadtimes are broken down
into reports about the time before the first task, time after the last task
and time between tasks, as well as the current report for all time spent
not processing tasks.

See merge request !359

Updated the cell sanitization tool to give more sensible values to the extreme h outliers.

That may help with the EAGLE 100 box. The idea is to limit the values of h in cells by creating a computing an upper bound and imposing it before the tasks get constructed. 

It gets activated if `InitialConditions:cleanup_h` is set to 1 in the YAML file.

See merge request !356

Mpi redistribute

Clean up the redistribute code to avoid excessive memory use and work around
an issue in Intel MPI that stops packets larger than 4Gb from being used
over Mellanox infiniband (actually that is dapl).

Memory use is reduced by separating the particle types into different sends
(so we don't have the overhead of a new copy of parts, xparts, gparts and sparts
all at the same time).


See merge request !345

Make sure gravity (self or external) is switched on when running with stars.

Fixes #320.

See merge request !358

Add missing mpoles policy name and fix maximum policy mask

Using -S ran off the end of engine_policy_names...

OK to delete branch.

See merge request !357

Using -S ran off the end of engine_policy_names

Add -P command-line option to overwrite or add program parameters

So you can run commands like:
```
 ../swift -s -t 4 -PTimeIntegration:time_end:1 -PDomainDecomposition:repartition_type:edge_weights sodShock.yml
```
and avoid editing the YML file. These parameters will overwrite any values already read
from the YML file or added new parameters as necessary.

See merge request !353

Add -P command-line option to overwrite or add program parameters without modifying the YML file, useful for quick tests

Disc patch revived

Implements the changes discussed in #311. 

 - The disc patch example is now 400x400x800 in length with correct internal energy.
 - The accelerations get truncated at |z-z_disc| > 300 with a cosine transition
 - The accelerations are 0 at |z-z_disc| > 380.
 - The calculation is significantly faster thanks to the extraction of constant terms.

See merge request !349

Reset the value of rho_dh to something sensible if no neighbours are found when imposing h=h_max

Fix to #314.

See merge request !351

More soft transition for the truncation of the potential. Set some plotting limits to the solution script.

Do not call the difffloat.py script with the additional deprecated parameters formerly used for rho_dh

Do not call the difffloat.py script with the additional deprecated parameters formerly used for rho_dh

Test periodic bc

This tests the periodic boundary conditions which are generated in `space_getsid()` and used in `runner_dopair1_density`.

An 8x8x8 mesh is generated that is periodic and 26 pair interactions are computed for the following cells:

* All 8 corner cells
* The cell at the centre of each face
* The cell at the centre of each edge

These checks are performed for both unperturbed and perturbed cells.

See merge request !350