trinary model extension

[YoelPH]

first push for trinary model extension. At this point the trinary model works, but is not generalized yet.

[rmnldwg]

Looks generally good! Here are some ideas to separate concerns a little more and generalize the code:

• [ ] fix #44
• [ ] replace keyword for modalities with dictionary of arrays in unison with clinical() and pathological() functions

[rmnldwg]

Hey Yoel,

I made some significant changes to the node.py and edge.py modules. The unilateral.py has some major changes, especially in its initialization, too.

As far as I was able to think in advance, the node.py seems finished to me and does not need any particular changes anymore. However, the two most important functions in the Edge class which compute the respective conditional probability are not yet implemented (search for the word "TODO").

In the unilateral.py there is a lot that still needs changes. Have a look at everything I changed. We can probably update more of the Unilateral class in a similar fashion.

Let's discuss this tomorrow. See you then!