remove/improve coverage limit of 254

[droeatumn]

The kmer counting program has a limit of 255. kmers occurring 255 or more are considered off-kir. In general, coverage shouldn't be too high, because then errors can start looking like non-errors. The work around is to down-sample the input data.

This limit should be addressed and removed. Ideally, it would learn the correct min/max counts based on the input.