Initialize a new graph for every instance of DotGenerator.

Allows for the generation of multiple graphs.
Move pydot logic out of dotgen to visualizer module.

Moves pydot logic to a separate module for now.

TODO: Discuss whether we want to have graphs generated directly to .png
(depends on graphiz).
Move visualize to separate test
Adding separate Visualize test.
Render AST image using tkinter
Update visualizer to draw parent pointers from children