Add interface for accepting spikes from an external source

[bcumming]

Take the interface and implementation that was written for the hackathon.

https://github.com/arbor-sim/arbor/tree/spike-external

[halfflat]

Having a look at what is required, I believe we need:

• The external_connections_ code in communicator.hpp (though I would keep the source unsigned).
• external_spike_callback in simulator and implementation class.
• Customizable min_delay — though I think this should be done at simulator initialization time rather than after.
• The make_event_queues overload in communicator.

I'd dearly like a less strongly-coupled solution for external spike exchange based on e.g. moving windows over per-simulator bundles of generated spikes and dynamic integration window size, but that's another story.

[halfflat]

Oh, and I think we should extend the external_spike_callback interface so that it takes an argument for the minimum window of external events, i.e. a t such that any subsequent call to the callback provides events with time not less than t.

We can check that assertion and throw an error indicating desynchronization.

[halfflat]

Might be a stretch goal, but I'd like to see this interface in the 0.2 release; adding the tag and we'll see how we go.

[halfflat]

Removing 0.2 tag; will aim for 0.3.

[thorstenhater]

🎉 closed by #2001