Port of Brunel example (#1404)

* Add Python version of brunel.cpp example.
* Enforce consistency in units: both C++ and Python APIs use unit `kHz` for all frequencies.
* Align Python version `poisson_schedule` signature with C++ equivalent.
* Update unit tests to suit API changes.
* Update documentation to suit, along with minor additions related to networks and interconnectivity.
* Add `brunel.py` to `basic.yml` tests