only create engine once for all tests
only create engine once for all tests

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