Add target_edges parameter to pegasus.find_clique_embedding

[JoelPasvolsky]

Current Problem
Unlike the Chimera version of find_clique_embedding, the Pegasus version requires a NetworkX graph.

Proposed Solution
Similar parameter to Chimera's find_clique_embedding

Alternatives Considered
Grunt work to use the existing function:

G =nx.Graph()
G.add_nodes_from(qpu.nodelist)
G.add_edges_from(qpu.edgelist)
p6 = dnx.pegasus_graph(6)
missing_nodes = [node for node in p6.nodes if node not in G.nodes]
missing_edges = [edge for edge in p6.edges if edge not in G.edges]
p6.remove_nodes_from(missing_nodes)
p6.remove_edges_from(missing_edges)

Additional context
Advantage

[boothby]

Preferred Alternative:

import dwave_networkx as dnx
import dwave.embedding as dwe
p = dnx.pegasus_graph(6, node_list = qpu.nodelist, edge_list = qpu.edgelist)
# even more preferable, already-requested feature:
# p = dnx.pegasus_graph(solver=qpu)
emb = dwe.pegasus.find_clique_embedding(k, target_graph=p)

[arcondello]

See also dwavesystems/dwave-networkx#165

[arcondello]

I think this has largely been resolved by the addition to to_networkx_graph() method.