Add Kobayashi neuron model

[apdavison]

Kobayashi R, Tsubo Y and Shinomoto S
(2009) Made-to-order spiking neuron model equipped with a
multi-timescale adaptive threshold. Front. Comput. Neurosci. 3:9.
http://dx.doi.org/10.3389/neuro.10.009.2009

mat2_psc_exp in NEST. Implementations for Brian and NEURON needed.

[appukuttan-shailesh]

NEURON implementation is available:
https://modeldb.science/226422

[HarshKhilawala]

@apdavison If this is pending, can you please assign this issue to me?

[HarshKhilawala]

@apdavison @appukuttan-shailesh Does Brian simulator have implementation for this model?