Should this also take into account any uncertainty the user has set in the underlying data? Suppose, for example, that the user has prepared the ``CCDData`` objects that are ffed into ``Combiner`` by doing ``create_variance`` for each, adding in the appropriate noise for each image -- then it seems like that should be reflected in the uncertainty that comes out of ``average_combine``.

It may be that we don't want to try changing this for now, but instead add it to a list of questions we want feedback on.