unittests for hardware backends

I have tested in these commits the different unittests files for the ESS backend. Particularly, I have added the test test_connectors_serial.
Note that I have, I think, corrected small bugs in common/population.

I hope these are useful.
If I understand correctly, this is added because the hardware neuron ids are not necessarily in increasing order. Isn't it possible to re-order the hardware ids?