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.