Minigraph-0.16 (r436)

Notable changes:

• Improvement: 2-level chaining. This is a feature backported from minimap2.
  It speeds up graph generation for human graphs.

• Improvement: break a chain at poorly aligned regions, another recent
  minimap2 feature.

• Added the script for generating figures in the minigraph paper.

(21 February 2022, r436)