Add models with NMDA dynamics according to Wang (2002)

[janskaar]

Update: Removed draft.

Adds iaf_wang_2002, a model with a conductance based exponential synapse with extra dynamics, from [1]. The model is simplified to allow efficient computation, by taking $t_\mathrm{AMPA,rise} \to 0$ and modelling it as a delta function. Since $t_\mathrm{AMPA,rise} \ll t_\mathrm{AMPA,decay}$, this should not make too much of a difference.

[1] Wang, X. J. (2002). Probabilistic decision making by slow reverberation in cortical circuits. Neuron, 36(5), 955-968.
DOI: https://doi.org/10.1016/S0896-6273(02)01092-9

[nicolossus]

@janskaar Thanks! Here is my initial review after looking at the implementation of the model.

[heplesser]

See my comments in the code, we can discuss directly later.

[heplesser]

Just one little suggestion now, still need to look at examples and tests.

Also consider to rename all _jump variables to _post, see notebook.