Chimera and Pegasus generators don't allow for Chimera/Pegasus indices when node/edge lists are passed in

[hhtong]

Description
chimera_graph() and pegasus_graph() have a parameter 'coordinates' that, if true, should mean that the node labels are 4-tuple chimera/pegasus indices. If false, they'll be linear integer indices.

This is currently not true when the user passes in node and edge lists (the node labels remain as linear integers even when coordinates=True). This seems like unwanted behaviour.

To Reproduce
The following two snippets will result in different indexing schemes.

            G = dnx.chimera_graph(*shape, 
                                  node_list=nodelist, 
                                  edge_list=edgelist,
                                  coordinates=True)

            G = dnx.chimera_graph(*shape, 
                                  coordinates=True)

Additional context
May want to also consider this adjacent issue: dwavesystems/dwave-system#289