Details:
newer versions of the root files also have 2D histograms
(e.g `foo_vs_meff`). These can appear only for a subset
of the signal regions, and therefore cause the check
`check_all_regions_have_the_same_number_of_histgrams`
to fail.
This fix prevents the 2D histograms from ending up in
the list of histograms to be merged.