Code review comment for lp:~ctjacobs-multiphase/fluidity/compressible-multiphase

Thanks for the feedback Stephan - improvements have now been made to Advection_Diffusion_CG.F90. I have decided to always set the 'multiphase' logical rather than rename it to something involving a specific equation type, since it might be used later on when adding multiphase support to the k-epsilon model.