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