One person has been killed in a traffic accident, in northern clarion county.    And, state Police identify the victim as 48-year-old James Eric Neiswonger of the Strattanville area.    Neiswonger was the operator of a motorcycle which was traveling on Route 66, in Farmington Township, about Nine Saturday night.    For unknown reasons, Neiswonger lost control of the bike.    The motorcycle veered off of the highway.    The bike went into a ditch and hit an enbankment.    The motorcycle went airborne and came to a stop over the bank.    Neiswonger was pronounced dead at the scene by the Clarion County Coroner.