Prepare for turning chordal_graph_cliques into a generator

[jarrodmillman]

See #4004.

[dschult]

I agree that a generator makes the most sense here.
Code like (from @jarrodmillman in #4004

for C in (G.subgraph(c).copy() for c in connected_components(G)):
        if C.number_of_nodes() == 1:
            yield frozenset(C.nodes())
        else:
            unnumbered = set(C.nodes())
            v = arbitrary_element(C)
            unnumbered.remove(v)
            numbered = {v}
            clique_wanna_be = {v}
            while unnumbered:
                v = _max_cardinality_node(C, unnumbered, numbered)
                unnumbered.remove(v)
                numbered.add(v)
                new_clique_wanna_be = set(C.neighbors(v)) & numbered
                sg = C.subgraph(clique_wanna_be)
                if _is_complete_graph(sg):
                    new_clique_wanna_be.add(v)
                    if not new_clique_wanna_be >= clique_wanna_be:
                        yield frozenset(clique_wanna_be)
                    clique_wanna_be = new_clique_wanna_be
                else:
                    raise nx.NetworkXError("Input graph is not chordal.")
            yield frozenset(clique_wanna_be)
and delete _connected_chordal_graph_cliques.

I believe this also makes the cliques deterministic. The connected components are ordered and while the nodes within each clique are not ordered, the cliques should be yielded in the same order. Nice!!

[jarrodmillman]

I already added the code from #4004 to a new internal function _chordal_graph_cliques and now chordal_graph_cliques is just the warning and

return {c for c in _chordal_graph_cliques(G)}

Before 3.0, we just need to replace chordal_graph_cliques with _chordal_graph_cliques.

@dschult If this looks OK to you, please go ahead and merge.