Simplify the access to the graph's nodes

[rlouf]

NetworkX's default is to separate the name of a node from the data attached to it; accessing a node by name G.nodes["n"] returns a dictionary with the data, here {"contents": Union[Var, Argument, RandVar, Transformation]}. As a result, the code is full of ["contents"] dictionary access calls, which is aesthetically unpleasant and unintuitive.

I think this behaviour can be overridden by subclassing the NodeView class in mcx.core.graphand overriding the graphs' nodes property.

This would greatly improve the code readability.