countVariableNodes
Returns the number of Variable nodes contained in a BinaryDecisionDiagram. Note that, since a node can be shared by multiple parents (e.g. in a reduced diagram built by BinaryDecisionDiagramBuilder.reducedOf), a shared node is counted once for every path that reaches it, not once overall.
Return
the number of variable-node occurrences reachable from this diagram.
Throws
if the traversal fails.