Move event wrangling from cell_group to model (#390)

* Remove `cell_group::enqueue_events` interface: `cell_group::advance` is given the events to deliver in the next epoch directly.
* Implement event lane merging in `model` with stand-alone `merge_events` function.
* Ensure event-delivery order is preserved across differing domain decompositions by performing a lexicographic sort over all of the `postsynaptic_spike_event` fields: fixes #385. 
                                                                                                         
Fixes #378.