The method pandas.DataFrame.sort_values replaced pandas.DataFrame.sort from pandas's version 0.17.0, and the call to the deprecated method was causing a bug preventing the writing of MuPeXI output files in environment with up-to-date versions of pandas (bug observed with pandas 0.20.1 but not with pandas 0.18.1).
The method pandas.DataFrame.sort_values replaced pandas.DataFrame.sort from pandas's version 0.17.0, and the call to the deprecated method was causing a bug preventing the writing of MuPeXI output files in environment with up-to-date versions of pandas (bug observed with pandas 0.20.1 but not with pandas 0.18.1).