Fix stereo calibration apps and python scripts
We assume that you are running on a bunch of images that are suitable.
So for chessboards, each image must have the right number of points, and be extractable. Same for rings. For AprilTags, you can have a varying number of tags per image, but there must be some.
And now, there must be at least 4 points in each view, and at least 4 common points across stereo left-right views.
Also, I updated a lot of doxygen documentation while doing this.