Consider making densratio.density_ratio.DensityRatio pickable.

[mierzejk]

Contingent upon data volume, as well as cross validation parameters, computing densratio.RuLSIF.RuLSIF function outcome can take significant time. Yet the result might be reusable, hence valuable. It would be nice if the resulting densratio.density_ratio.DensityRatio object could be pickable, according to What can be pickled and unpickled? Currently there is a non top-level function that is a member of the object's, so trying to dump the pickle brings about the Can't pickle local object 'RuLSIF.<locals>.alpha_density_ratio error.