nmodl export: Passing events to child elements is not supported

[epiasini]

This is needed for properly simulating synaptic plasticity, as in NeuroML a plastic synapse relays incoming spiking events to its (child) plasticityMechanism which uses spike timing information to calculate the plasticityFactor.

[epiasini]

Some ideas about how to do this:

• it should be possible to set up a system where the synapse uses properly tagged self-events to communicate with its child, mimicking the parent-child distinction even though they are both living in the same mod file;
• alternatively, the case for plastic synapse could be hardcoded and self-events avoided, but this would be less flexible with regards to future extensions of the specification (think STDP or LTP).