Added a small syntax-sugar function overload for cv::parallel_for to all...

...ow passing lambda functions as parallel loop body.

This function allows to apply parallel computing like this:

cv::parallel_for(cv::Range(0, evolution_.size()), [&evolution_](const cv::Range& range) {

    for (int i = range.start; i < range.end; i++) {
        // Compute multiscale derivatives for the detector
        compute_scharr_derivatives(evolution_[i].Lsmooth,evolution_[i].Lx,1,0,evolution_[i].sigma_size);
    }

});