Add baryonic matter density paramter and its evolution to cosmology classes

I tried to rename branches, in the process deleted and unintentionally closed #2757. This  branch started from a current upstream master and has all the commits I made merged by hand. I have no idea why all these other commits show up again and I clearly don't have the time to figure this out myself. I'll be happy if you can make this work but I will do some science now.