Use a more uniform folder structure

[D-K-E]

A more uniform folder structure is needed. The suggested structure is:

• pygmodels/
  • graph/
    • func/
    • type/
    • model
  • factor/
    • func/
    • type/
    • model
  • pgm/
    • func/
    • type/
    • model/

The idea is that type/ contains base type and abstract object and the base object that implements its methods. model/ folder contains objects that inherit from base class. func contains functions that act on both stuff in types and also in model.
By convention the *ops module in func/ should apply to base class.