Petri graph layout clean new branch

[marjoleinpnnl]

Merges jc-graph-refinments with Petri graph with fixed layout with update so it keeps all moved nodes in a fixed position and double clicking unfixes the node. Petri graph is viewable in notebook/visual examples/Graphs.ipynb

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[JosephCottam]

Added tests.
Changed variable names in the graph schema to clarify their role.
Removed a toggle in the schema whose action was hard to understand and whose purpose was difficult to imagine beyond testing.

[SamWitty]

@JosephCottam and @marjoleinpnnl , I'm going to revert this PR and restore the branch because of the conflicts with the major refactor going on in staging-refactor. I'm mostly doing this so that the work great work here doesn't get omitted from the next version of PyCIEMSS. I should have the staging branch merged in very soon, so hopefully this wont need to hand for long.