Clarified cosmology dependence in morris av. Closes #568

This fixes #568 (closed).

New version behaves exactly the same as previous one, but with more sensible cosmology factors.

Just to confirm, it actually breaks the test if you miss an a-factor in there?

Yeah, it does. Here's it ran with a instead of a^2.

Looks like a difference with little consequences on the physical properties.

merged

mentioned in commit 2e08ae0c