Extract the log prior probability from a model.
Extract the log prior probability from the first model saved in a
a valid BayesMfp-Object of length 1 (otherwise only first element recognized)
Note that the returned value might be only defined up to a constant, because some models might not be identifiable or have a dimension too large, and are thus implicitly assigned a zero prior probabibility.
Daniel Saban\'es Bov\'e