Add greyscale colormap to 2d viewer

[DanicaSTFC]

• closes Add color bar to registration viewer #384
• the visibility of the bar is in sync with the buttons
• the rendering of the range is in sync with the translation values
• the window level etc are not in sync. add observer

[paskino]

I would have thought that this would be sufficient.

Suggested change

	data = self.subtract.GetOutput()
	# Retrieve the scalar data from the output
	scalars = data.GetPointData().GetScalars()
	# Check if scalars exist in the data
	# Get the min and max scalar values
	cmin, cmax = scalars.GetRange()
	v = self.vis_widget_reg.frame.viewer
	# make sure the stats are updated
	v.ia.Update()
	cmin = v.ia.GetMinimum()
	cmax = v.ia.GetMaximum()

I don't see you connected any observer?

[DanicaSTFC]

07/07/2025

We will not have time to finalise the observers. Danica is going on leave. Ideally, we would want to add this feature in the CILViewer. We are pausing this PR for the moment.

Edo is keener to have the #169 instead of simply this.