[Request] Add RMSE calculation to GCPs

[mradamcox]

Describe the feature or enhancement you're looking for
Somehow, somewhere, a calculation for Root Mean Square Error (RMSE) should be added to the interface, so that some estimation can be visualized for the proper fit of control points. It would be nice to have indicators in the interface that show the residual error of each point, like QGIS does.

Additional context
I don't know how to do this; it will be a good challenge. Also, RMSE does not indicate the accuracy of the georeferenced map, only how distorted the image is after it has been warped by the provided control points. This is useful with large-scale maps that are drawn very accurately, because they ultimately should have very little distortion. However, it is useless with Thin Plate Spline, because that algorithm always produces 0 RMSE for every GCP.

As such, this ticket requires a lot more research before implementation.