Probabilities in post-processing

[JohannesBuchner]

At the moment, the API allows per-catalog "magnitude" histograms for weighing posterior probabilities.
NWAY could allow multi-catalog band combinations to be passed to a function for reweighing. This would allow ruling out / down-weighing unphysical / unlikely magnitude/color combinations.

To do so, before https://github.com/JohannesBuchner/nway/blob/master/nwaylib/__init__.py#L107 the table should be passed to a user-defined function, which should returns another biasing log-bayes factor for each row, which is added to total, just before _compute_final_probabilities is called.