Add example demonstrating spike-based simulations of Pong

[krachbumm3nte]

A NEST-based simulator for training spiking networks to play the classic game Pong, based on: https://github.com/electronicvisions/model-sw-pong

Permission to use part of the original code in the NEST repository has been granted by Timo Wunderlich and Eric Müller (representing "Electronic Vision(s)")

setting as draft until #2384 is merged

[terhorstd]

@krachbumm3nte, thanks for also adding this example. Just FYI: I just merged https://github.com/nest/nest-simulator/pull/2384, unblocking this one.

[JanVogelsang]

Wouldn't have thought that PyCharm doesn't indent lines according to PEP8, thanks for correcting the indentation. Looks good to me now!