APE allow for vaiable speed of sound

This MR turns the base density rho0 into a field to allow for spatially varying rho0 and changes the APE implementation to match the original APE1/4 LHS which allows for correct treatment of rho0 and p0 gradients. All test and example cases as well as the documentation were adjusted in order to reflect these changes. Existing constant p0/rho0 cases are not affected.