RT testing example: RandomizedBox_3D

This MR introduces a new small example for RT testing.

This test case contains only gas and stars in a periodic box. The contents have no physical meaning, and are background particles on a regular grid overlapping a randomly sampled sine wave particle distribution. The purpose of this test are as follows:

• Neither the stars nor the gas particles are updated all at the same time every step. This test has commonly particles being updated at their individual time step. So you can run with an unmodified version of swift to get individual star time steps.
• The stars always have neighbours, so that's one less thing to thing about when testing your schemes.
• Contrary to examples/RadiativeTransfer/UniformBox_3D, the test runs on gadgetoid cosmological internal units, not on cgs.