vtkCurveRepresentation is a new abstract base class for
representations of curves such as vtkSplineRepresentation and
vtkPolyLineRepresentation.

The bulk of the implementation has been moved to
vtkCurveRepresentation. vtkSplineRepresentation and
vtkPolyLineRepresentation have been made subclasses of
vtkCurveRepresentation.

Change-Id: Ic9f8c70ecafff1e90bfea4d19aafb9fe7419d9f8