Performance improvements in ensemble.py
Performance improvements in ensemble.py

mostly by hiding debug logging behing if statements (checking
whether to report at the debug level was expensive in inner loops)

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