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.