populations + getindex/iteration support

[wsphillips]

Population allows creating arbitrary numbers of neurons from a prototype/template using the Base.Iterators interface.

This PR will also implement iteration and indexing on MultiCompartmentSystems (to index subcompartments versus calling by name with getproperty) and the same for NetworkTopology and NeuronalNetworkSystem.

[wsphillips]

Enables convenient syntax like:

for neuron in network, compartment in neuron
    # process each compartment of each neuron in a NeuronalNetworkSystem
end

# make a synapse between two neurons
topology[pre, post] = Glut(30nS)
# alternatively...
topology[pre => post] = Glut(30nS)

# mixed syntax...
network[2].soma # soma of the 2nd neuron in the network