Python spikes (#788)

Support for recording spikes generated by a simulation in the Python wrapper
* Implement a `spike_recorder` that holds a shared pointer to a `std::vector` of spikes, and a callback for the `arb::simulation` spike recording API.
* Add `python/example/ring.py` that creates a ring network, then records and prints spikes.
* Some fixes to get the full `recipe` -> `domain_decomposition` -> `simulation` -> `spikes` workflow to work
  * always use default `global_parameters`: user customization of global parameters for cable cells can wait until the ion species interface is finished.
  * change the Python recipe interface for `recipe::connections_on` to use `pybind11::objects` because of shim.
* Some small improvements to error and help messages.

Fixes #764