Performs OLS

        Returns
        ----------
        None (stores latent variables)

def _ols_fit(self):
        """ Performs OLS

        Returns
        ----------
        None (stores latent variables)
        """

        # TO DO - A lot of things are VAR specific here; might need to refactor in future, or just move to VAR script

        method = 'OLS'
        self.use_ols_covariance = True
        
        res_z = self._create_B_direct().flatten()
        z = res_z.copy()
        cov = self.ols_covariance()

        # Inelegant - needs refactoring
        for i in range(self.ylen):
            for k in range(self.ylen):
                if i == k or i > k:
                    z = np.append(z,self.latent_variables.z_list[-1].prior.itransform(cov[i,k]))

        ihessian = self.estimator_cov('OLS')
        res_ses = np.power(np.abs(np.diag(ihessian)),0.5)
        ses = np.append(res_ses,np.ones([z.shape[0]-res_z.shape[0]]))
        self.latent_variables.set_z_values(z,method,ses,None)

        self.latent_variables.estimation_method = 'OLS'

        theta, Y, scores, states, states_var, X_names = self._categorize_model_output(z)

        # Change this in future
        try:
            latent_variables_store = self.latent_variables.copy()
        except:
            latent_variables_store = self.latent_variables

        return MLEResults(data_name=self.data_name,X_names=X_names,model_name=self.model_name,
            model_type=self.model_type, latent_variables=latent_variables_store,results=None,data=Y, index=self.index,
            multivariate_model=self.multivariate_model,objective_object=self.neg_loglik, 
            method=method,ihessian=ihessian,signal=theta,scores=scores,
            z_hide=self._z_hide,max_lag=self.max_lag,states=states,states_var=states_var)