Add properties for interface sets in network
Add properties for interface sets in network

* interface_set_ensembles (dict)
* transition_interface_sets (dict)

Both of these will, I think, be part of generalizing the new
analysis approach and making networks easier to create and use.

jhprinz/openpathsampling,dwhswenson/openpathsampling,jhprinz/openpathsampling,choderalab/openpathsampling,choderalab/openpathsampling,dwhswenson/openpathsampling,dwhswenson/openpathsampling,choderalab/openpathsampling,openpathsampling/openpathsampling,jhprinz/openpathsampling,dwhswenson/openpathsampling,openpathsampling/openpathsampling,openpathsampling/openpathsampling,openpathsampling/openpathsampling