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.
By looking back at the commit comments: Before my change, the connector fails to connect two populations if index 0 of a population connects to index 0 of the target population, and if this connection is the only connection. 
ANother reason for the change is that the connection was producing an error if the mask was set to "all false".
I will write a test for both cases