Optimize Python scripting to share engine instances and encapsulate script scopes

[glopesdev]

Original report by me.

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[glopesdev]

Original comment by Eirinn Mackay (Bitbucket: zouden, GitHub: zouden).

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+1, this would be a huge improvement.

Currently I'm trying to get bonsai to control a web-based presentation system using selenium. The problem is that each time selenium is instantiated it creates another browser window (this is impossible to avoid according to the selenium devs). So in Bonsai, every time I make a change to the Python code, a new window appears. The solution would be to have one node that initializes the selenium object, and use other python nodes to interact with it. Currently this is not possible as nodes have a separate python instance.