Create TreeBN class

Most of the code available in [BinaryCLT](https://github.com/deeprob-org/deeprob-kit/blob/main/deeprob/spn/structure/cltree.py) actually works for any tree-shaped Bayesian Network. Therefore, it would be better to create a super-class called TreeBN and then make BinaryCLT a subclass of it.




