Updated the way we converge towards the smoothing length. We now use a Newton-Raphson scheme. This applies to the Gadget-2 scheme.

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

Be more conservative when splitting self-task into subs. We don't want to trigger a rebuild straight after.

Modified the splitting rule for self tasks to be less conservative. Added proper documentation of the splitting conditions.

Also create functions to decide when to rebuild and when to split a task. Update the rebuild and split condition to correctly use kernel_gamma.

Add new function to decide on recursion with in a sub_pair task. Makes sure all recursion tests are identical.

Completes d8025de1

Use dx_max_sort_old and not dx_max_sort when checking whether cells have been sorted in DOPAIR1_BRANCH()

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