Runtime variable changes aren't reset

[maxmaynard]

When a graph is executed, any variable/set changes alter the JSON format with the new value; that value doesn't reset when the graph is run again, or changes are made, etc. This took me a day to realize and I feel that this could either be communicated more clearly, or, when the graph is changed or play is pressed again, the variable is reset to the initial value.

[mattmacf98]

For now, I documented that in the README and an associated variable-behavior.md, I think it would be good to have a "reset variables" button at some point. I'll leave this open until I can get around to that