Add export functionality for 3D points and a visualization tool

[abrock]

I implemented #43 (first commit). I think the visualization of error directions and magnitude is great and I wanted to use it for visualizing results from other calibration methods. I also wanted to use different color scales for the visualization so I added methods to export Image to text files I can process with gnuplot, and Image<Vec2f> to optical flow files in the Middlebury format (https://vision.middlebury.edu/flow/data/).

[puzzlepaint]

Thank you very much for the PR. I will have a look at it when I have time.