Cosmological version of the standard Sod shock test.

In the co-moving coordinates that SWIFT uses, the Euler equations of 
hydrodynamics have an elegant form with a few additional factors that 
involve the scale factor a. For the specific case of a polytropic index 
gamma = 5/3, all additional factors are in fact the same: 1/a^2. For 
this case, hydrodynamics in the co-moving frame is identical to 
hydrodynamics in a physical non-cosmological frame with a rescaled time 
variable dt'=a^2*dt.

We choose an Einstein-de Sitter cosmology with H(a)=H_0*a^(3/2) and a 
box size of 1 pc and rescale the Sod shock initial conditions so that 
the internal coordinates, density and pressure values are still the same 
as for the original Sod shock in the non-cosmological case. We then 
evolve the initial condition from z=999 to z=960.5, which corresponds to 
a rescaled time interval dt'~0.12. If the co-moving coordinates are 
implemented correctly, the resulting co-moving density, internal 
velocity and co-moving pressure profiles should match those for the 
non-co-moving variables in the ordinary non-cosmological Sod shock at 
t=0.12.