OpenMMEngine: use masses from mdtraj topolgy directly if system (and therfore forcefield) are not know
OpenMMEngine: use masses from mdtraj topolgy directly if system (and therfore forcefield) are not know

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