Added documentation to the Subdiv2D documentation, based on the previous
C API.

fixes #5689

Rewrite the note on HoughCircles documentation to make it more clear

Add note to clarify that the output vector of found circles is sorted by
descending order of centres accumulator values.

Also delete reductant lines on the HoughCircles documentation.

Added comments to hough circles function.

Added comments to icvhoughgradient

Misalignment in line 1183 corrected

- disabled doxygen tree
- enabled doxygen enum listing
- added imgproc reference to main page
- enabled BiB support
- chenged doxygen root page format

Add Triangle method for automatic threshold computation next to the existing
Otsu's method. Triangle deals better with images whose histogram does not
contain dominant peak.

See paper Zack GW, Rogers WE, Latt SA.: Automatic measurement of sister
chromatid exchange frequency. J Histochem Cytochem. 1977 Jul;25(7):741-53.

Fixed doc for Bug #3887

(cherry picked from commit b027eac1)

Apart from not displaying the table, it breaks the doc build of https://github.com/Itseez/opencv_contrib/pull/70.

removed optim module; moved its functionality to core and photo modules; moved drawing functions from core to imgproc. Removed FilterEngine etc. from public API

Corrected crashing in Python bindings while using LSD_REFINE_NONE flags
Corrected not drawing lines in drawSegments and compareSegments in Python bindings

Updating missed defines in documentation. The new defines (in imgproc.hpp) state they have a prefix of COLOR instead of CV.

Clarify how the mask parameter is set on output and how this is
affected by the flags parameter.

resolves Feature #2942

Fixed spelling of CV_INTER_LINEAR flag in geometric_transformations.rst