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.
Presumably there is a lot of overlap between this and the renamed `test_connectors_parallel.py`. It would be better to to merge these into a single file, and have each test cover both cases, choosing the comparison data accordingly. Having two separate, highly similar files risks becoming a maintenance nightmare.