TreeToHepMC confuses milibarn and microbarn

### Environment: (where does this bug occur, have you tried other environments)
All

### Steps to reproduce: (give a step by step account of how to trigger the bug)
1. Convert a EicTree root file to HepMC using TreeToHepMC

### Expected Result: (what do you expect when you execute the steps above)
crossSection gets converted from microbarn to HepMC's standard picobarn

### Actual Result: (what do you get when you execute the steps above)
TreeToHepMC false assumes that the input crossSection is in milibarn, and is thus wrong by a factor of 1000

Offending code: https://github.com/eic/eic-smear/blob/master/src/erhic/TreeToHepMC.cxx#L130-L153