Compute the expectation:
    <m(X)>_p(X)
        - m(x) :: Linear, Identity or Constant mean function

    :return: NxQ

def _expectation(p, mean, none1, none2, none3, nghp=None):
    """
    Compute the expectation:
    <m(X)>_p(X)
        - m(x) :: Linear, Identity or Constant mean function

    :return: NxQ
    """
    return mean(p.mu)