add transformed values to the Pst.parameter_data attribute

def add_transform_columns(self):
        """ add transformed values to the Pst.parameter_data attribute

        """
        for col in ["parval1","parlbnd","parubnd","increment"]:
            if col not in self.parameter_data.columns:
                continue
            self.parameter_data.loc[:,col+"_trans"] = (self.parameter_data.loc[:,col] *
                                                          self.parameter_data.scale) +\
                                                         self.parameter_data.offset
            #isnotfixed = self.parameter_data.partrans != "fixed"
            islog = self.parameter_data.partrans == "log"
            self.parameter_data.loc[islog,col+"_trans"] = \
                self.parameter_data.loc[islog,col+"_trans"].\
                    apply(lambda x:np.log10(x))