Add new polyline decimation strategies

The vtkDecimatePolylineFilter has been refactored to use a strategy pattern. It used a default strategy based on eulerian distances. This change brings 2 new available strategies :

• vtkDecimatePolylineAngleStrategy which uses the angle between 3 consecutive points to perform the decimation.

• vtkDecimatePolylineCustomFieldStrategy which uses a specified PointData array to gather the values used for the decimation.

It is also possible to make custom strategies by inheriting the vtkDecimatePolylineStrategy class.

The new default strategy for the vtkDecimatePolylineFilter is the vtkDecimatePolylineDistanceStrategy which implements the old eulerian distance computation.