Barrelcortex example (showing standalone issue)

In general this is a nice example that now seems to work (it is not *exactly* like the Brian1 example and there were some inconsistencies in the Methods description in the paper but in first approximation, it all looks good). It uses spatial connectivity, scalar variables etc. and is therefore a nice showcase for Brian2, I think. In contrast to Brian1, it doesn't use a single loop to set up the network or analyze the results :)

Standalone compiles and runs as well, but **it does not give the same results**. The pinwheel-map structure that is visible in runtime is not there, so apparently the learning didn't do what is supposed to be. I have another example of an STDP network (written by a student in Berlin) where standalone compiles (and runs very fast) but the results are very different from the runtime and do not make much sense.

If possible, I'd like to release an alpha without this discrepancy... I didn't do any detailed debugging yet, the synapse creation seems to yield the correct number of synapses so I rather think something goes wrong during the simulation.