1D advection test for radiative transfer.
This test is identical to the examples/RadiativeTransferTests/Advection_1D
with the difference that particles aren't equally spaced on a line, but
separated into three regions with decreasing intervals between particles
in each of the region.

More concretely, for a box of size 1, the separation is given as

dx      if x < 0.33
dx/2    if 0.33 < x < 0.66
dx/4    if x >= 0.66

This leads to the three regions having different time step sizes for RT.


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-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

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.