Generic method for computing the score of a "dbn" S3 object.
Calls bnlearn's score
underneath.
The nodes function is specifically coded to look for either the "bn"
or the "bn.fit" method inside bnlearn, so I have to cast the "dbn" class
into "bn" for it to work.
# S3 method for dbn
score(object, ...)
the score of the network
a "dbn" object
additional parameters