`fd.TemplateSource.simulate` bug when `multihist.axis_names` not properly defined

[plt109]

Describe the bug and how to reproduce

Simulating events from a fd.TemplateSource constructed with a 3d multihist returned MC events in a dataframe that has only 2 dimensions. This is most likely because the axis_names of the 3d multihist was poorly defined, although it is indeed a list of length 3.

Expected behavior

Simulating events from a fd.TemplateSource constructed with a 3d multihist but with axis_names with no empty strings indeed returned MC events with the correct number of observables.

[JelleAalbers]

Thanks Pueh! Yes, this would happen if axis names are duplicated (or maybe just if they are empty). We could add a check for this and throw an error if this happens.