RBR iterator

Adds a helper function to be able to iterate over DAQ/MC events easily.

Example:

julia> rbr = RBRIterator(f)
RBRIterator(ROOTFile{OnlineTree (11405 events, 11405 summaryslices), OfflineTree (11405 events)})

julia> @showprogress 1 for (evebt, mc_event) in rbr
            # ...
       end
Progress: 100%|███████████████████████████████████████████████████████████| Time: 0:00:21