code: def to_parfiles(self,prefix):
          
          if self.isnull().values.any():
              warnings.warn("NaN in par ensemble",PyemuWarning)
          if self.istransformed:
              self._back_transform(inplace=True)
  
          par_df = self.pst.parameter_data.loc[:,
                   ["parnme","parval1","scale","offset"]].copy()
  
          for real in self.index:
              par_file = "{0}{1}.par".format(prefix,real)
              par_df.loc[:,"parval1"] =self.loc[real,:]
              write_parfile(par_df,par_file)
docs: write the parameter ensemble to PEST-style parameter files
  
          Parameters
          ----------
          prefix: str
              file prefix for par files
  
          Note
          ----
          this function back-transforms inplace with respect to
          log10 before writing