`Graph` visualizations improvement
Created by: MorrisNein
- Refactored default graph representations by matplotlib + networkx.
- Added 'pyvis' visualization engine for interactive graph views via html.
- Added 'graphviz' visualization engine (requires Graphviz and pygraphviz).
- Matplotlib visualization supports nodes_size and edges_curvature arguments.
- All graph and history visualizations now support
dpiargument.