Variable AV fix | Cosmology

Fixes the AV and AD coefficients in ANARCHY and in the Morris & Monaghan AV. Thanks to @bvandenbroucke for the Cosmological Sod Shock tests, they were instrumental in debugging this.

Before fix; artificial diffusion coefficient as a function of space:

After fix:

I am currently testing this fix with the EAGLE_12 to see if it rectifies the problem we were having there. Once that is over (I guess whether it passes or not), I'll re-assign this and remove the WIP.

doc/RTD/source/GettingStarted/running_example.rst

@@ -11,7 +11,7 @@ as ``wget`` for grabbing the glass).
 
 .. code-block:: bash
    
-   cd examples/SodShock_3D
+   cd examples/HydroTests/SodShock_3D
    ./getGlass.sh
    python makeIC.py
    ../swift --hydro --threads=4 sodShock.yml