potential speed-up in reading ROOT files

[KrisThielemans]

https://root.cern.ch/doc/master/TEntryList_8cxx_source.html says

 __GetEntry(n)__ - returns the n-th entry number
- __Next__()      - returns next entry number. Note, that this function is
                much faster than GetEntry, and it's called when GetEntry() is called
                for 2 or more indices in a row.

We're using GetEntry at the moment. There's maybe an easy work-around to have a method that check if we should call Next or GetEntry depending on the number (seems a bit silly that ROOT wouldn't do that).

The alternative might be to just call Next, as we will always process in sequence (except when using the save_position).