Implement a generic N(z) loader

[arthurmloureiro]

Firecrown currently has factories to generate the SRD distributions but, in the future, we will want to load N(z)s in different ways.

I think these are the scenarios we are most likely to encounter.

To do:

• Load N(z)s from SACC if these are present
• Load N(z)s from other files
• Generate generic Smail N(z)s with generic Photo-z gaussian error from the input Yaml
• (for the last two cases): save the generated N(z) in the SACC file

Not sure the last one needs to be implemented outside of Firecrown or if it would be best to liaise with Firecrown devs to implement it there.