Fix Networks, implement NetworkSystem

[wsphillips]

Network is no longer working after the updates to ConductanceSystem and CompartmentSystem. It should be fixed by writing a NetworkSystem that similar implements the ModelingToolkit system interface.

The default constructor for NetworkSystem should support taking a Metagraph or similar datastrcuture capable of describing network topology, neuron types, synapse types and weights.

For now we will create a directly coupled system of equations. Future features may include spike-triggered discrete synaptic events.

[wsphillips]

NeuronalNetworkSystem now implemented by #18

Support for graph objects as input punted to #24