Cleverer ids for particle splitting

Two updates:

• Add the option to remap all the particle IDs to [1, N] after having read the ICs,
• Add the option (set as default) to use the next available IDs when splitting particles rather than random ones that can potentially create conflicts.

This is useful for the set of EAGLE runs we want to do.

@pdraper any thoughts on the implementation are welcome.

If/when we are all happy, I'll modify all the relevant parameter files and update the documentation.

doc/RTD/source/CommandLineOptions/index.rst

@@ -34,6 +34,7 @@ can be found by typing ``./swift -h``:
     -s, --hydro                       Run with hydrodynamics.
     -S, --stars                       Run with stars.
     -B, --black-holes                 Run with black holes.
+    -k, --sinks                       Run with sink particles. 
     -u, --fof                         Run Friends-of-Friends algorithm to
                                       perform black hole seeding.
     -x, --velociraptor                Run with structure finding.