[RF][HS3] Export binned RooDataSets just like the RooDataHists

Check if any exported RooDataSet actually represents a binned dataset,
and then import it like a RooDataHist. This happens frequently when
people create combined RooDataSets from binned data to fit HistFactory
models. In this case, it doesn't make sense to export them like an
unbinned dataset, because the coordinates are redundant information with
the binning. We only do this for 1D data for now.