Now it's legal_replicas and select_sample.
Starting PathMovers support for ens and rep vars

(In initialization.) Also added the make_list_of_pairs function, which will
be used to clean input for swap-like moves. Starting to implement
EnsembleHopMover. First step toward using a SampleSet in the calculations,
too.
Fix syntax error; add None to make_list_of_pairs
Draft of EnsembleHopMove
Start conversion of bootstrapping to new version