How to getcenter of vtkplane by python?

How to getcenter of vtkplane by python?
@lassoan@Juicy@jamesobutler @jcfr @pieper

Add half of the origin->point1 and origin->point2 vector to the origin to get the center.

I would recommend to use the markup plane node. It allows you to display and interactively modify the plane and has convenience functions for setting/getting the center position.