Calculate the abundance distribution of k-mers from a single sequence file.

Note that with -b this script is constant memory; in exchange, k-mer counts will stop at 255. The memory usage of this script with -b will be about 1.15x the product of the -x and -N numbers.

The columns of the k-mer abundance histogram are: (1) k-mer abundance, (2) k-mer count, (3) cumulative count, (4) fraction of total distinct k-mers.

(from the khmer project: http://khmer.readthedocs.org/en/v2.0/ )