Complete background calculations

[nikfilippas]

This PR adds the following background functions:

• hubble_distance(cosmo, a)
• comoving_volume_element(cosmo, a)
• comoving_volume(cosmo, a, *, solid_angle=4*pi)
• lookback_time(cosmo, a)
• age_of_universe(cosmo, a)

and with that closes #954 and closes #408 and now CCL will be able to calculate every distance quantity described in Hogg's reference manual .

[damonge]

This is not to be reviewed until current functionality has been put in v3 form. There are several other new science implementations that take priority.