`internal_edges` function

[mtfishman]

This is a good idea for a generic graph function. It is the complement of the edge boundary of a subgraph (https://en.wikipedia.org/wiki/Boundary_(graph_theory) and boundary_edges added in ITensor/NamedGraphs.jl#20).

As we discussed, a simple implementation would be internal_edges(g::AbstractGraph, vertices) = edges(subgraph(g, vertices)). Then this function could be defined as internal_edges(P::AbstractProjTTNO) = internal_edges(underlying_graph(P), sites(P)).

Originally posted by @mtfishman in #18 (comment)