added: main_featureDetect.cpp

Allow feature detection parallelization using GNU parallel

    Usage: ./openMVG_main_featureDetect [OPTIONS...] -o <outdir> <image> ...
    
    OPTIONS:
    -o|--outdir <path>           Destination directory
    -r|--distratio <ratio>       Distance ratio (default=0.6)
    -s|--octminus1 [0|1]         When set to 1, SIFT will upscale the image x2
    -p|--peakThreshold <thresh>  Peak threshold for SIFT, 0.04 (default) to 0.01

Example:

     cd images
     cat ../matches/lists.txt | cut -f 1 -d ';' | parallel -j<number_of_jobs> main_featureDetect -o ../matches