Retina (and SpikeSourceArray) edges always routing entire source spectrum

[dkfellows]

See SpiNNakerManchester/sPyNNakerExternalDevicesPlugin#43

If I create a big population of IF neurons and project it to an array of smaller populations this creates many individual edges which are typically as wide as the address space of the neurons on the respective source subvertex. Although not optimal, because the target cores might receive packets which it does not care about, this is typically acceptable.

For virtual cores/chips like the PushBotRetinaDevice, though, this leads to the horrible situation in which all spikes generated by the retina are sent to all cores which receive a projection from the retina - regardless of the size of this projection.