mvd: bugfix in rounding the station Z in the negative Z region
Bugfix in rounding the MVD station Z. The calculation was wrong for negative Z. ( Why should the Z position be rounded anyhow? )
The bug affects the new shifted CBM geometry.
Due to the bug, all nominal positions of the MVD stations, returned by
CbmMvdDetector->GetParameterFile()->GetZPosition(iStation)
were shifted from the mvd material by 2 cm.