Fix a(t) in cosmo

I am glad to have made this overflow because I found another bug. Now we have a good accuracy (about 1e-8) with this method :)

I have made some changes in order to make the code more easy to read and understand

Fixes #450 (closed).

changed the description

changed the description

Thanks! All good for me.

merged

mentioned in commit 0fd491d2

mentioned in merge request !1591 (closed)