quickly corrected the previous refactoring of features2d: moved from set(SOME_PROP, val) to setSomeProp(val)

1. someMatrix.data -> someMatrix.prt()
2. someMatrix.data + someMatrix.step * lineIndex -> someMatrix.ptr( lineIndex )
3. (SomeType*) someMatrix.data -> someMatrix.ptr<SomeType>()
4. someMatrix.data -> !someMatrix.empty() ( or !someMatrix.data -> someMatrix.empty() ) in logical expressions

Added sample of work of Fisheye::undistortImage and its description to documentation. Removed readPoints and readExtrinsic (useless)

Added function decomposeHomographyMat. New files added are homography_decomp.cpp and test_homography_decomp.cpp.
Modified files calib3d.hpp and camera_calibration_and_3d_reconstruction.rst.

Conflicts:

	modules/core/src/stat.cpp