Implement cell-local event generators (#414)

Implement generic `event_generator`, and some common generators that should be useful to users out of the box:
* poisson sequence                                                                                       
* uniform regular sequence
* from a user-prescribed sequence
* from a `std::vector` of post synaptic events.

Extend `recipe` API to provide an `recipe::event_generators(gid)` that returns a list of `event_generator`s attached to a cell with `gid`.

Update the `model` class and `merge_events` method to support event generators.

Fixes #401