False alarm probability, used to compute the prior on N_{\rm
blocks} (see eq. 21 of Scargle 2012). If gamma is specified, p0 is
ignored.
ncp_prior : float (optional)
If specified, use the value of ncp_prior to compute the prior as
above, using the definition {\tt ncp\_prior} = -\ln({\tt
gamma}). If ncp_prior is specified, gamma and p0 are
ignored.