Notes about Brian on GPU
========================
Thresholding and compaction
---------------------------
This can be quite slow. To make it faster, one option if there is a refractory
period of at least R time bins and there is a minimum synaptic transmission delay
of at least R is to calculate spiketimes every R time bins, and feed them to
the Synapses object.

This means that when a spike is produced, its timing is stored.

