hal2vg v0.1: Original hal2sg-based logic

This is a snapshot of the hal2vg logic as it was before being completely re-written to use Cactus's Pinch Graph library.

It works reasonably well on small graphs, and could be useful to sanity check the newer logic. It also provides various subsetting and filtering options that aren't yet available in the new version.

But.. it scales very badly, especially in the degree of the HAL tree nodes, making it unusable for a lot of the whole-genome data we're now interested in.