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 purposes of this test are
as follows:

- Neither the stars nor the gas particles are updated all at the same time. 
  This test has commonly particles being updated at their individual time step.
- The stars always have neighbours, so that's one less thing to thing about when
  testing your schemes.
- Contrary to ../UniformBox_3D, the test runs on gadgetoid cosmological internal
  units, not on cgs.

You can and should run the same sanity check scripts as in ../UniformBox_3D to
check that everything's right.

Compile swift with:
    --with-rt=GEAR_1 --with-rt-riemann-solver=GLF --with-hydro-dimension=3 --with-hydro=gizmo-mfv --with-riemann-solver=hllc --with-stars=GEAR --with-feedback=none --enable-debugging-checks --with-grackle=$GRACKLE_ROOT



Some specific notes for debugging:

-   when running on 2 MPI ranks (at least in the first few steps) cells 27 and
    151 have proxies on the other rank. However, those are made for the gravity
    tasks, not for the RT tasks. So their foreign counterparts will have no RT
    tasks at all. (run with debug-RT scheme and 4 sub-cycles. To catch the error,
    remove additional check in cell_is_rt_active() to see whether cell actually
    does RT.)
