Update lensing likelihood

[mgerbino]

We should update the lensing likelihood:

• move theory calculations (e.g., addition of noise controbutions) in separate blocks (e.g., cobaya theory blocs)
• use sacc format for data products, and add related routines for I/O
• use sacc format for corrections components
• make computation of fiducial cls optional in case we want to read fiducial from file