Fixed exceptions, print statements, and long types for gen_pattern.py to be Python 3 compatible (#7994)

* fixed exceptions and print statements to be python 3 compatible; removed long type checks (py3 uses int); whitespace reformatting

* Pulled latest svgfig from upstream

https://github.com/jpivarski/svgfig/commit/f3179a8926508bf0f9021fd4e1f9731c95524a38

Add OpenCV parallel_for_ tutorial.

Use directly the doxygen snippet command for the Changing the contrast and brightness of an image tutorial. Extend the tutorial with a pratical example and with a gamma correction method to adjust the brightness of an image.

Add check for all zero moments. If one of the shapes is empty, the match would return zero distance between the shapes even when the other one had content. It now returns DBL_MAX if no moments had value.

Modified the (R2,T2) computation documentation in the stereoCalibrate function documentation to display on two lines for clarity

Fix Documentation Typos (#7942)

Append zero to trailing decimal place for FileStorage JSON write of a float or double value (#7952)

* Fix for FileStorage JSON write of a float or double value that has no fractional part; appends a zero character after the trailing decimal place to meet JSON standard.

* strlen return to size_t type rather than unnecessary cast to int

Change `<=` to `<` to avoid accessing the first character of an empty string

3.2.0