get the maximum likelihood parameter estimate.

        Returns
        -------
         post_expt : pandas.Series
            the maximum likelihood parameter estimates

def mle_parameter_estimate(self):
        """ get the maximum likelihood parameter estimate.

        Returns
        -------
         post_expt : pandas.Series
            the maximum likelihood parameter estimates
        
        """
        res = self.pst.res
        assert res is not None
        # build the prior expectation parameter vector
        prior_expt = self.pst.parameter_data.loc[:,["parval1"]].copy()
        islog = self.pst.parameter_data.partrans == "log"
        prior_expt.loc[islog] = prior_expt.loc[islog].apply(np.log10)
        prior_expt = Matrix.from_dataframe(prior_expt)
        prior_expt.col_names = ["prior_expt"]
        # build the residual vector
        res_vec = Matrix.from_dataframe(res.loc[:,["residual"]])

        # calc posterior expectation
        upgrade = self.mle_covariance * self.jco.T * res_vec
        upgrade.col_names = ["prior_expt"]
        post_expt = prior_expt + upgrade

        # post processing - back log transform
        post_expt = pd.DataFrame(data=post_expt.x,index=post_expt.row_names,
                                 columns=["post_expt"])
        post_expt.loc[islog,:] = 10.0**post_expt.loc[islog,:]
        return post_expt