move spiked detection to back end (#167)

Moves spike detection to the back end, which is required to have fast spike detection on the gpu.

Fixes #106.

Backend:
* Move spike detection into the backend.
* Add a `threshold_watcher` type to each backend that is initialized with a reference to the field that it is to watch, along with indexes and thresholds for each variable in the field that is being watched. This class presents a simple host-side interface:
   * `test(t)` tests for crossings at time `t`.
   *  `crossings()` returns all threshold crossings.
   * `clear_crossings()` resets (clears) the collected crossing events.
* Implement the multicore back end detector directly from the original code.
* Implement a `gpu_stack` for use with the gpu back end, that lets threads in a kernel conditionally push back into a flat array.

Miniapp:
* Run a single-step dummy run of model before starting the profiler, when profiling is enabled.
* Initialize the spike output callback functions _after_ t...