Implementation of asynchronous integration API. (#210)

* Stage events for next integration interval on lowered cell.
* Use explicit binning for event coalescence.
* Extend `event_queue` to allow checking top of queue against arbitrary predicates.
* Add `--bin-dt` and `--bin-regular` options to miniapp (disable binning with `--bin-dt 0`).
* Tidy up miniapp option settings class.

Integration in lowered cell over multiple steps is deferred until samplers can be set up with back-end polling.

Asynchronous integration itself is not yet implemented.