Interface harmonization of in-memory (vg.cpp) and on-disk (index.cpp) graph representations about vg HOT 1 OPEN

@adamnovak @glennhickey @edawson and I discussed this on a call the other day.

We should establish a list of functions that we'd like to have any vg-style object support. This should be pretty easy now that we've made five or six implementations of vg-handling libraries. If we can do this then the vg API can be implemented abstractly, and flexibility provided by giving different storage interfaces to the same system. For example, an xg-backed VG object should look basically the same as one that's backed by hash tables, although it couldn't be writeable. So perhaps there should be static and dynamic versions of the API, with the static one a read-only subset of the dynamic.

from vg.