Cobaya connector: support more variables

[heatherprince]

Usually in cobaya I can include a bunch of relevant variables in my chain output files even if they aren't being sampled over (e.g. I can get cobaya to save the value of sigma_8 for each step in the chain even if I am sampling As instead). This didn't work with a firecrown likelihood. It would be really helpful to regain this functionality and be able to sample and save a wider range of parameters with cobaya.

[vitenti]

@heatherprince In Firecrown one can use DerivedParameters (as in the des_y1_3x2pt example) to include extra information computed during the likelihood computation back in the MCMC chains. Please take a look at cobaya_evaluate.yaml:171 where we inform cobaya to include that parameter in the chains. The same appears in the cosmosis example des_y1_3x2pt.ini:20.

However, the example you gave (sigma8), that is computed by CAMB and can be included in the chains without touching Firecrown (in cobaya you simply include sigma8 in the list of derived parameters).

Is that what you need?