Compute the expectation:
    expectation[n] = <m1(x_n)^T m2(x_n)>_p(x_n)
        - m1(.), m2(.) :: Constant mean functions

    :return: NxQ1xQ2

def _expectation(p, mean1, none1, mean2, none2, nghp=None):
    """
    Compute the expectation:
    expectation[n] = <m1(x_n)^T m2(x_n)>_p(x_n)
        - m1(.), m2(.) :: Constant mean functions

    :return: NxQ1xQ2
    """
    return mean1(p.mu)[:, :, None] * mean2(p.mu)[:, None, :]