rGFA output

[edawson]

Because we use a reference genome for input, we can easily generate rGFA. This also gets around the many issues of representing paths in GFA 1 and GFA 2.

We need only three tags to make minigraph-compatible rGFA output:

Tag	Type	Description
SN	Z	Name of stable sequence from which the segment is derived
SO	i	Offset on the stable sequence
SR	i	Rank. 0 if on a linear reference genome; >0 otherwise

For reference graphs, this is pretty simple. We'll need to implement pulling some type of VCF name in as well though.