1D advection test for 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!

The ICs are created to be compatible with GEAR_RT and SPHM1RT. Recommended configuration:
GEAR_RT (with gizmo-mfv solver):
    --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

GEAR_RT (with sphenix SPH solver):
    --with-rt=GEAR_3 --with-rt-riemann-solver=GLF --with-hydro-dimension=1 --with-hydro=sphenix  
     --with-stars=GEAR --with-feedback=none --with-grackle=$GRACKLE_ROOT

SPHM1RT:
    --with-rt=SPHM1RT_4 --with-hydro-dimension=1 --with-stars=basic  --with-sundials=$SUNDIALS_ROOT
    
IMPORTANT: Need SUNDIALS version  = 5 . 
SUNDIALS_ROOT is the root directory that contains the lib and include directories, e.g. on cosma:
SUNDIALS_ROOT=/cosma/local/sundials/5.1.0/

Note that in SPHM1RT any (SPH) hydro scheme is compatible.

Note that if you want to use a reduced speed of light for this test, you also 
need to adapt the fluxes in the initial conditions! They are generated assuming
that the speed of light is not reduced.