Petri graph layout clean new branch (#391)

* Multi-graph special handling for 'all'.  More robust simple spring-force graph.

* set layout in graph

* any changes?

* all moved nodes stay stuck

* change variable name

* unfix by double clicking

* add layout input as argument in function rather than just the signal boolean

* Fixing node-color-scale reference.

* Graph layout passed in as dictionary-of-points (networkx convention)

* Removed 'layoutdata' flag

* retaining input x/y more clearly

* whitespace cleanup

* Improved names for x/y coordinate values.

* Example notebook

* Added tests for graph layouts.

* Future work around graph labeling.

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Co-authored-by: Joseph Cottam <[email protected]>