General HebbianModel refactoring

[Pibborn]

I am not very happy about the current state of HebbianModel.py.

1. Not enough separation between high-level code (referring to the hebbian model and its object attributes) and low-level code (involving class dictionaries and so on)
2. the evaluate() function is a mess.
3. there must be some other way to produce plots than the one that is currently in place.

These feel like three separate issues, but I probably need to rewrite the evaluation code in general. I feel it is kind of OK if we just want to be done with the current experiments, but I would not feel comfortable releasing it to the open.

[Pibborn]

I am kind of happier now that commit 7b20b64 is in, but I am going to keep this open. I was a bit harsh on evaluate(.) and I don't really see a way to achieve 1., but 3. is definitely OK now.