This enables ddsim to accept `--inputFiles example.hepmc3.tree.root` files as produced by the [HepMC3 rootIO writers](https://gitlab.cern.ch/hepmc/HepMC3/-/tree/master/rootIO/include/HepMC3).

There are two rootIO writers: `WriterRoot` writes HepMC3 objects to a ROOT file, while `WriterRootTree` writes a ROOT TTree with events. There are two corresponding rootIO readers. When HepMC3's `deduce_reader` is passed a `.root` file, it only [attempts to use](https://gitlab.cern.ch/hepmc/HepMC3/-/blob/master/include/HepMC3/ReaderFactory.h#L107) the `WriterRootTree`.

Since `.root` is a bit too generic as an extension (it does not indicate the data model) and since ddsim already uses multiple extensions in output formates, e.g. `.edm4hep.root`, I considered that the intent is best conveyed with `.hepmc3.tree.root` (making it clear that the HepMC3 data model is used, stored in TTrees, inside the ROOT file format).

With this change it is now also possible to run ddsim against remote HepMC3 events on an xrootd server, e.g.
```
ddsim --compactFile $DD4hepINSTALL/DDDetectors/compact/SiD.xml -N 10 --inputFiles root://dtn-eic.jlab.org//work/eic2/users/wdconinc/pythia8NCDIS_5x41_minQ2\=1_beamEffects_xAngle\=-0.025_hiDiv_vtxfix_1.hepmc3.tree.root --outputFile output.edm4hep.root
```

DDSim: compactFile can only be one instance, otherwise we cannot overwrite what is in the steeringfile, still takes multiple XML files

From https://stackoverflow.com/a/41153081

Needed for python2.7 compatibility

The XML files need to follow the structure where at the end is a closer file.
compactFile takes multiple arguments, or multiple instances of compactFile or both.

  ddsim --compactFile $DD4hep/examples/CLICSiD/compact/SiD_multiple_inputs.xml $DD4hep/examples/CLICSiD/compact/SiD_detectors_1.xml --compactFile $DD4hep/examples/CLICSiD/compact/SiD_detectors_2.xml $DD4hep/examples/CLICSiD/compact/SiD_close.xml --runType shell