add keywords to sort_index call
add keywords to sort_index call

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