Add massive nu

[ASKabalan]

This pull request adds support for massive neutrinos to the Cosmology class in jax_cosmo, allowing for more accurate modeling of cosmological parameters. The main changes include introducing the Omega_nu parameter throughout the class, updating serialization logic, and adding new default cosmologies that include neutrino mass.

This is needed since in CosmoGrid simulations Omega_c + Omega_b does not add up to be Omega_m without the Omega_nu parameter

[santiagocasas]

I can review this PR in 1-2 days. It's just a "background" addition of the massive nu, right? Also, this does not contain the "m_nu" parameter. One could add a simple camb-like conversion between m_nu and Omega_nu, to start with.

[ASKabalan]

I can review this PR in 1-2 days. It's just a "background" addition of the massive nu, right? Also, this does not contain the "m_nu" parameter. One could add a simple camb-like conversion between m_nu and Omega_nu, to start with.

Hello @santiagocasas

Yes it is just a background addition since in Cosmogrid simulations Omega_c and Omega_b do not add up to be Omega_m unless we include Omega_nu

I will see how to add the conversion

[ASKabalan]

Also Planck15 and Planck18 parameters are to be checked

Probably from
2015 : https://arxiv.org/pdf/1502.01589
2018 : https://arxiv.org/abs/1807.06209