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