Also move cv::linemod to own header

The program would crash if release is called twice.

This issue seem to be lingering around for quite some time
https://github.com/Itseez/opencv/pull/639
http://code.opencv.org/issues/2819

Also, removed the one from modules/python/src2/cv.py and cleared its
executable bit, since it's not a script.

This allows subclasses of CvBoost to override initial weights that
update_weights uses without duplicating its entire implementation.