First try of efficient factor operations
Modified factor multiplication and added factor division.

* resulting variables of multiplication are now sorted
* added factor division
* added tests for multiplication and division
Moved from relative to absolute imports
Added __str__ for factor.
Variable values are specified during creation.
Classes for network nodes.
Added observation to variable nodes.
Solved sending messages
Implementation of basic message passing strategy.
Reworked multiplication and division of factors and tests.
Modified nodes, added directional edges.
Reworked nodes, improved tests, added normalization.
Fixed lbp.
log arithmetic used for factors.
Added normalization and changed probs.
added abstraction
Added list of variables to init
Modified observations andd node neighbors.

Observed can now be more than one value.
Neighbors are not distinguished between incoming and outcoming.
Edges are not directed now. Added lbp for singly linked graphs
Removed directory structure