ZGroupCurvesDataFrame class inherits class data.frame.
Class does no implement new methods, but it is used to override some
behaviour such as plot. Usually ZGroupCurvesDataFrame object belongs
to a Zresults object.
If user modifies or subsets ZGroupCurvesDataFrame in a funtion it is up
to the user to update the indexes in slot is.group
is.group:Logical indicating whether column is actually a group.