Allow simulator controls/processing plug-ins

[cmeyer]

Allow scripts to add stem simulator controls and provide camera specific code to apply them to the simulated image.

[Brow71189]

This is already mostly possible, but a bit finicky to use. You can do stem_controller.create_control() and stem_controller.add_contro() to create and add a new control. Linking them to existing controls is really difficult and we should probably provide some methods to make that easier.