Fake sampler

Add a "fake" sampler that reads parameters from a specified posterior file and evaluates the likelihood at these parameters. The results object is updated with these newly computed likelihood values.

One use case for this fake sampler is to test likelihood accuracy for ROQs, by comparing the full and ROQ likelihood at the same parameter space points.

I also added the set_strain_data_from_zero_noise method to InterferometerList for convenience.