1D advection test for cosmological radiative transfer.

Test that your method is TVD and the propagation speed of the photons is
correct. The ICs set up three photon groups: 
- The first is a top hat function initial distribution where outside values
  are zero.
- The second is a top hat function initial distribution where outside values
  are nonzero. This distinction is important to test because photon energies 
  can't be negative, so these cases need to be tested individually.
- the third is a smooth Gaussian. 

This way, you can test multiple initial condition scenarios simultaneously. 
There are no stars to act as sources. Also make sure that you choose your
photon frequencies in a way that doesn't interact with gas!

There are 3 parameter files available:
- rt_advection1D_low_redshift.yml for a test at very low redshift (almost no cosmological expansion)
- rt_advection1D_medium_redshift.yml for a test at moderate redshift (z=10)
- rt_advection1D_high_redshift.yml for a test at high redshift (z=110)

Select one and give it to the run.sh script via command line argument:
    ./run.sh rt_advection1D_[redshift_of_your_choice].yml


The ICs are created to be compatible with GEAR_RT and SPHM1RT. Recommended configuration:
GEAR_RT:
    --with-rt=GEAR_3 --with-rt-riemann-solver=GLF --with-hydro-dimension=1 --with-hydro=gizmo-mfv \
     --with-riemann-solver=hllc --with-stars=GEAR --with-feedback=none --with-grackle=$GRACKLE_ROOT