'Returns the working covariance or correlation matrix for a
given cluster of data.
Parameters
endog_expval: array-like
The expected values of endog for the cluster for which the
covariance or correlation matrix will be returned
index: integer
The index of the cluster for which the covariane or
correlation matrix will be returned
Returns
M: matrix
The covariance or correlation matrix of endog
is_cor: bool
True if M is a correlation matrix, False if M is a
covariance matrix'

def covariance_matrix(self, endog_expval, index):

    raise NotImplementedError
