Allow neurons composed of multiple compartments

[wsphillips]

We should be able to connect together assemblies of compartments. These types of neuron models follow the form of those similar to:

Pinsk & Rinzel 1994 -- two compartment model

Davison et al 2000 -- four compartments

and range up in size to (for instance) the classic 19-compartment Traub et al 1992 model.

These types of models could be implemented by defining a function that specifies equations for the conductance between individual compartments and returns a new system using compose from MTK. Additional tooling to handle compartment-specific synaptic conductances and stimuli will also be necessary (e.g. post-synaptic connection targeted to Neuron Y, compartment Y)