Builds on core class updates described in other pull request. Refactored graph viz modules, fixed minor bugs in core classes

[IsaacFigNewton]

• Renamed core class files according to PEP 8 standard
• Reintroduced add_attr_to_X_metadata to HypergraphI for backwards compatibility
• Added DOT conversion for Hypergraphs
• Rewrote/simplified Chaikin Smoothing function, merged Object class into HypergraphVisualizer class
• Added tests for hypergraph visualization modules

[IsaacFigNewton]

Also, maybe for future work, but the bounding areas representing hyperedges in the HypergraphVisualizer module could be made more robust with something like what's described here, to avoid accidentally including vertices that might not in a given hyperedge, but close to it. Before Chankin smoothing I mean.