ENH: Add function to get parameters from stimuli, which you can use to recreate it

[TEParsons]

The main use of this is ioHub - because it runs in a separate process, we can't pass object handles directly, so have to recreate any stimuli. Making all stimuli serializable means we can pass a JSON string between processes and use it to recreate the stimulus on the other end, meaning any stimulus is then able to use used in calibration/validation, not just TargetStim.