Option to mark a GBZ as a subgraph of another

[jltsiren]

Changelog Entry

To be copied to the draft changelog by merger:

• vg gbwt option --subgraph-of for marking a GBZ graph a subgraph of another.

Description

This adds vg gbwt option --subgraph-of for marking a GBZ graph a subgraph of another. It requires that the supergraph has already had its pggname (stable graph name) set.

Usage example:

# Set pggname in the supergraph, if necessary
vg gbwt --set-pggname -Z supergraph-input.gbz -g supergraph-output.gbz

#Set pggname and a subgraph relationship in the subgraph
vg gbwt --subgraph-of supergraph-output.gbz  -Z subgraph-input.gbz -g subgraph-output.gbz