Uncertainties calculation for omnidir calibration can be incorrect

[savuor]

System information (version)

• OpenCV => 3.4
• Operating System / Platform => all
• Compiler => all

Detailed description

Related to #19803@main

According to the [MVG], various least squares method explanations (like this) and the standard camera calibration code in OpenCV (here), the parameters uncertainty should be calculated with the "minus k" denominator correction:
$$\sigma^2 = \frac{1}{n - k} \sum^n_{i=1} \left( x_i - \bar{x} \right)^2$$
where k is a number of non-fixed model parameters.

The incorrect calculation appears in two places in omnidir calibration:

• cv::omnidir::internal::estimateUncertainties(): here
• cv::omnidir::internal::estimateUncertaintiesStereo(): here

MVG: R. Hartley, A. Zisserman, Multiple View Geometry in Computer Vision, 2004, section 5.1.3, page 134

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues,
  forum.opencv.org, Stack Overflow, etc and have not found any solution
• I updated to the latest OpenCV version and the issue is still there
• There is reproducer code and related data files: videos, images, onnx, etc