Runs a uniform box with radiation initially present. No further radiation 
sources are present, and the gas should heat up and ionize, while the
radiation fields should decrease at the same rate.

To run with GEAR-RT, compile swift with:
    --with-rt=GEAR_3 --with-rt-riemann-solver=GLF --with-hydro=gizmo-mfv --with-riemann-solver=hllc --with-stars=GEAR --with-feedback=none --with-grackle=$GRACKLE_ROOT