insufficient refactoring when cov_params_default was added
if cov_p is None and self.normalized_cov_params is None:
raise ValueError('need covariance of parameters for computing '
short-circuit if no arguments are given and we just want cov_params(). There is no need for checking all cases if all arguments are None.
I assume we don't have a subclass yet that does not define normalized_cov_params, so no error or failure so far.
there is not really much to short_circuit, it just goes through a few if and ends up returning cov_p
PR in #1772
REF: cov_params allow case of only cov_params_default is defined closes