+ Included conversion from `pybind11::object()` to `arb::util::optional` for `arbor.proc_allocation().gpu_id` and exception handling for `arbor.context(mpi=comm)`.

+ cleaned-up `test_event_generator.py`

+ for discussion: How do we want to handle the strings? Include them in appropriate `.cpp` or collect them in `strings.c/hpp`?