Replace vl-hog with a modern C++ solution

[patrikhuber]

vl-hog has really good performance and it's hard to replace. But I don't like to include C headers and I would really like to replace it with a header-only, modern C++ solution. It's just hard to find one, and rewriting vl-hog would be a lot of work.

I managed to include vl-hog as "header-only" by #include'ing vl-hog's .c file, and I had to make some casts in its code explicit in the process to get it to compile. I hope what I did is not problematic.

Did you try the opencv HOG implementation ? since you already use opencv...

[patrikhuber]

I've had quite bad performance with OpenCV's HOG implementation when I tried it back then. It was either accuracy, speed, or both.