Added FileNodeIterator for OpenCV 4.X to optimize vocabulary loading

[yhabib29]

Since OpenCV 4.0.0, the core module has been completely reimplemented in C++. see https://github.com/opencv/opencv/wiki/ChangeLog#version400

Iterating throught FileNode without iterator has become much more slower. To compare, see test_filenode program in branch yhabib29/DBoW2#dbg-opencv4.

Average time for each loop iteration in function load in TemplatedVocabulary.h:1454:

• Without iterator: 1,618,788 ns.
• Using iterator: 895 ns. Tested on vocabulary ORBvoc.yml (download link) with OpenCV 3.4.2 and 4.3.0 .

[yhabib29]

Duplicate of #68, sorry I did not see the PR