Implement a generate theory-vector for 3x2pt using SRD distributions

[arthurmloureiro]

We need to be able to generate a theory-vector from the Yaml API for 3x2pt.

As a first step, using the built-in SRD N(z)s and later on implement a more generic interface for using any N(z).

These theory vectors should also be saved in SACC format.

To-do:

• Yaml interface for getting Cls with RSD N(z)s
• Yaml interface for getting 2PFC with RSD N(z)s
• Save them in SACC format:
  • add a "generated by CosmoAPI tag" to the metadata
  • Save also the N(z)s in the SACC format
• Command-line interface to generate these data-vectors

Note that this should ONLY use firecrown!