Return view from schedule, replace time_seq. (#536)

Reduce redundant functionality across event_generator, time_seq
and schedule by providing a low-heap overhead interface to schedule
and using that for time sequences in event_generator and specialized
cell groups.

* Have schedule return pair of pointers as view to generated times.
* Fix missing DEBUG/TRACE functionality.
* Use rate instead of mean_dt for Poisson schedule.
* Move merge_events() functionality to simulator.cpp.
* Migrate event_generator to event span interface.
* Migrate tourney_tree to event span interface.
* Only invoke tourney_tree merge if generators have events in the epoch.
* Use schedule for times in event_generator implementations.
* Replace seq_generator with explicit_generator that keeps a copy of events.
* Replace vector_backed_generator and poisson_generator with schedule_generator.
* Replace time_seq uses with schedule.
* Add default empty schedule.
* Move rounding error test for regular time sequence into schedule test.
* Update sampling API documentation for schedule.