Add PGManifest interface/link/lan query/iterator methods
I'd like to add some sugar methods/data structures to the PGManifest object to make it easier to iterate over the experiment network metadata, or query it. I would also add some best-effort, non-fatal code that would help it find the "real" interface associated with a mac addr/ip addr, if the geni-lib script is being run on the node associated with that mac. The presence of good code like this would have made the Openstack profile's scripts much easier to develop.
I have a little of this code already, but I would expand it.
(Partly closes #34 (closed))