Added replica_list and ensemble_list methods
Added replica_list and ensemble_list methods

These return all the replica IDs and all the ensemble names, respectively,
by just returning the keys to the appropriate dictionaries.

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