Do not massively generate code strings for testing which will have to be all compiled individually when using weave. Should remove the recent massive slowdown of `test_synapses.py`. Also add tests for `N_incoming` and `N_outgoing` to `test_subgroups.py`.
Speed up synapse creation in numpy by only doing one bincount outside of the loop. Also move `_owner.resize` out of the loop.
Change the implementation of `N_incoming` and `N_outgoing` to properly work with subgroups.