Deprecate libbdsg odgi

[adamnovak]

The odgi project's "odgi" graph is now substantially different than the libbdsg version, and they've never quite been serialized-data compatible, and the odgi tool wants to use GFA for interchange anyway. Also, libbdsg's odgi has outstanding issues (#19) and isn't really maintained, because all the real odgi graph data structure development happens in the odgi project.

We should deprecate libbdsg's odgi graph implementation.