Generic method for obtaining the names of the nodes in "dbn" S3 object.
Calls bnlearn's nodes
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
nodes(object, ...)
the names of the nodes
a "dbn" object
additional parameters