add example on optional JAPC

it should explain how to wrap a class that either takes a real JAPC object or a fake one.

Rules for coi.SingleOptimizable:

• render must return static number of figures
• figures created with matplotlib.figure.Figure()
• actors must be in range [-1, +1]
• metadata = {'render.modes': ['matplotlib_figures']}
• metadata must declare JAPC usage -> if optional, two subclasses

added documentation enhancement labels