fix some pep8 complaints openmm/engine.py
fix some pep8 complaints openmm/engine.py

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