Implement the Poon-Domingos SPN structure

[RenatoGeh]

Description: Implement the Poon-Domingos SPN structure for local dependency as described in [1] and [2]. Rename learn/poon.go to model/poon.go, since it is not a learning algorithm, but a modeling for images and any local dependency dataset.

File: model/poon.go

References:

• [1] http://spn.cs.washington.edu/papers/spn.pdf
• [2] http://papers.nips.cc/paper/4544-learning-the-architecture-of-sum-product-networks-using-clustering-on-variables.pdf