Generics to support spike-driven affects for CompartmentSystem state

[wsphillips]

This issue is a key part of the work necessary for solving #61

Provide a keyword argument for defining additional affects that modify the state of presynaptic CompartmentSystem following a spike-triggered event. The default use case is for voltage resetting (e.g. in LIF neurons). This will let us move away from using "baked-in" model-specific constructors.