GBWTGraph v0.1

This is the first standalone release of GBWTGraph. The main features are:

• handlegraph::HandleGraph and handlegraph::SerializableHandleGraph interfaces.
• A version of follow_edges() that only follows paths supported by the haplotypes.
• Direct access to the internal sequence representation.
• Graph construction from GFA 1.0 with no overlaps/containments and integer segment identifiers.
• Minimizer index construction from the haplotypes.

This release requires GBWT v1.0.