Add AdEx cells.

[thorstenhater]

Add Adaptive Exponential Cells to Arbor. Motivation here is to add another useful cell kind before considering two major features:

1. Branching out Cell Group support by offering a plugin interface like the mechanism ABI. Finding a proper abstraction should be easier by having 3+2 examples instead of 2+2.
2. Adding support for custom synapse dynamics on point models.

Logically though, 2. should go before 1. since a plugin interface should declare how and if custom synapses are handled...

Closes #1832

More on request.

[ErbB4]

looks good and I will come back to comment again after I use it.