Register the event for a left button down click

def left_button_down(self, obj, event_type):
        """Register the event for a left button down click"""
        # Get 2D click location on window
        click_pos = self.iren.GetEventPosition()

        # Get corresponding click location in the 3D plot
        picker = vtk.vtkWorldPointPicker()
        picker.Pick(click_pos[0], click_pos[1], 0, self.renderer)
        self.pickpoint = np.asarray(picker.GetPickPosition()).reshape((-1, 3))
        if np.any(np.isnan(self.pickpoint)):
            self.pickpoint[:] = 0