Step printer crashes when next volume is undefined

[EinarElen]

The step printer code assumes that the track has both a current and a next volume. When this isn't the case, we get a nullpointer dereference

[EinarElen]

This happens if you are looking at a particle that is in the World_PV. The fix is trivial

// Before 
  auto nextVolume{track->GetNextVolume()->GetName()};
  // After (or something similar)
  auto nextVolumePtr{track->GetNextVolume()};
  auto nextVolume{nextVolumePtr ? nextVolumePtr->GetName() : "undefined"};