Add a sacc-less utility "likelihood"

[tilmantroester]

Right now the likelihoods in firecrown require a sacc object to compute anything. It would be nice to have a utility likelihood that takes in a list of ell (or theta) values, a collection of n(z), what kind of statistic should be computed, and then assembles a corresponding sacc object internally. That likelihood would then just compute the predicted data vector and not implement compute_chisq. The use case I have in mind is model exploration and testing, for which a measured data vector and covariance is not necessary and adds an unnecessary burden on the user to set up a sacc object themselves.