New Exercise: Decision Making Circuit (Wong & Wang)

New exercise

http://neuronaldynamics-exercises.readthedocs.io/en/latest/exercises/perceptual-decision-making.html

In the exercise "Perceptual Decision Making" we study an implementation of the circuit proposed in by Wong, K.-F. & Wang, X.-J. A Recurrent Network Mechanism of Time Integration in Perceptual Decisions. J. Neurosci. 26, 1314–1328 (2006)

Bugfixes

a few minor fixes