number of prior information equations

        Returns
        -------
        nprior : int
            the number of prior info equations

def nprior(self):
        """number of prior information equations

        Returns
        -------
        nprior : int
            the number of prior info equations

        """
        self.control_data.nprior = self.prior_information.shape[0]
        return self.control_data.nprior