added cached parameter to read() and readfp(). updated main() to cache the data immediately for better speed review.
updated README.md, we don’t need scan 0.4.7 anymore because we dropped the use of scan.Scanner.read_n(). scan 0.4.6 or some versions lower should be fine as well.
updated nas.py main(), added -p/—-progress option.
added 20MB test scene.