Change how access to the graph node features is handled

[StevenMaio]

Right now, access to the node features is done via the FixPropRepairLearn class. And so all of the classes which need access to the node features need access to FPRL. In order to keep the signatures the same, I added a _fprl field to all of these classes. However, this is horrendous. And so I'd like to change this at some point.

I think there are two approaches to this:

• some type of decorator pattern
• or I can extend the Model class to include the graph (which I think is probably easier)

I recommend to my future self to go through the second route.

[StevenMaio]

I submitted a change which handles this