removed all the hardcoded symlinks to full pleiades images.

Now each stereo dataset contains one or two text files containing the
relative paths to the panchro and/or multispectral images. The symlinks
have to be generated by the user using the script 'create_links.sh'
located in the 'pleiades_data' folder.
add shell script to generate the symlinks to the pleiades data
cleanup
added rpc data for all the available pleiades datasets
corrected the 'create_links.sh' script and updated README
added a few debug lines in the filter_matches_epipolar_constraint function
replaced the sift keypoints matching step with Ives' binary from his Ipol
paper, to use the relative threshold instead of the absolute one.
added a python function to evaluate the pointing error in a ROI of a Pleiades
image.

Removed the restriction to the first 1000 sift points.
saved in a txt file the pointing error measured on each dataset
corrected wrong paths to some pleiades images
add safe_image_zoom