edit-distance based block splitting

This allows splitting out sequences from POA blocks that are too distant from each other. Pairwise identity is estimated (crudely) based on edit distance, and alignments are greedily clustered with the first group of sequences that they have a match in.

initial release

This tool is starting to stabilize. Important features are coming into place and minor bugs with the initial implementation have been resolved.

We now have both smoothed GFA and MAF output.

Adaptive banded global POA (abPOA) lets us compute the smoothed graph efficiently.