include _t<n> suffix in check for existing files

[ManuelHu]

fixes #197

for root files with ntuple merging, there might still be false-positives. But we do not care that much about root files at the moment :-)

[tdixon97]

Should we just stop supporting the ROOT file output option, just another thing to maintain..

[ManuelHu]

We do not really "do" anything to support root. It would take more code for us to disallow root files...

[gipert]

I think we should keep it, if it's easy to maintain. I expect a lot of non-LEGEND users would be happy to keep all their ROOT-based post-processing infrastructure (thinking about screening facilities).

[ManuelHu]

This is remarkably complex, and needs more investigation... (apparently, there is no way to abort the run from the worker. And at the master BeginOfRunAction, the application cannot not know the number of threads...)