Use Graphviz for client-side auto layout

[astoeckel]

This issue is to document my crazy idea to use graphviz for improved auto-layout in nengo GUI. This would have some advantages:

• Possibly better layout for hierarchical networks
• Generated dot graph descriptions could be re-used to generate high-quality network renderings for publications, etc.

So my proposal would be the following:

• When the user clicks the auto-layout button, on the client-side, translate the graph datastructure to an appropriate .dot representation (in memory). The same .dot file can be easily offered for download as well (e.g. via URL.createObjectURL)
• Pass the .dot file to a stripped-down graphviz running on the client (the WASM version), tell graphviz to generate layout data only (plain format)
• Turn the layout data into the nengo-gui SVG representation

Graphviz is licensed under a permissive licesne (Eclipse Public License), so this shouldn't be an issue.

See [1] for a WASM/asmJS version of graphviz and [2] for a demo. Some functionality can surely be thrown away to reduce the binary size (at the moment 700kB asmJS, WASM is probably 2-3 times smaller)
[1] https://github.com/mdaines/viz.js
[2] http://viz-js.com/

[jgosmann]

Somewhat related (in so far as it was also an attempt to use GraphViz, but not inside the GUI): nengo/nengo#524