'Used to determine the confidence intervals for the variance.
It calls test_var and when called by an optimizer,
finds the value of sig2_0 that is chi2.ppf(significance-level)
Parameters
var_test : float
Hypothesized value of the variance
Returns
diff : float
The difference between the log likelihood ratio at var_test and a
pre-specified value.'

def _ci_limits_var(self, var):

    return (self.test_var(var)[0] - self.r0)
