RT: first example

This merge request adds a trivial test case for the RT debug scheme: swift needs to be compiled with --with-rt=debug. The example generates initial conditions with uniformly distributed hydro particles and a smaller, also uniformly distributed, star particle box in the center such that the hydro particles close to the faces of the box have no star neighbours.

Furthermore, the run is set up such that each time step all particles are active, and each time step an output is generated. This is necessary for the examples/RadiativeTransferTests/rt_uniform_box_checks.py script to run successfully.

The examples/RadiativeTransferTests/rt_sanity_checks.py script tests some general properties of the RT debug scheme, and any simulation run with this particular RT scheme should pass the tests therein.

The examples/RadiativeTransferTests/swift_rt_debug_io.py contains IO routines for the debugging output that is used by the other scripts.

Tl;dr:

• examples/RadiativeTransferTests/rt_uniform_box_checks.py only works for this example
• examples/RadiativeTransferTests/rt_sanity_checks.py is made to work on any run with the RT debug scheme

Note: The python scripts will fail/not work until !1323 (merged) is merged.