'Finding the root of sum(xi-h0)/(1+eta(xi-mu)) solves for
eta when computing ELR for univariate mean.
Parameters
eta : float
Lagrange multiplier in the empirical likelihood maximization
Returns
llr : float
n times the log likelihood value for a given value of eta'

def _find_eta(self, eta):

    return np.sum(((self.endog - self.mu0) / (1.0 + (eta * (self.endog - self.mu0)))))
