Generic method for obtaining the names of the nodes in a BN or a DBN.
Calls bnlearn's nodes
underneath.
I have to redefine the generic and mask the original for it to work on both
bn and dbn objects without the user having to import bnlearn.
nodes(object, ...)
the names of the nodes
a "bn", "dbn", "bn.fit" or "dbn.fit" object
additional parameters