Add unique _ptr to the model class (will be refactored later to have a manager)

* fixed const interface to the serial proxy for thread private storage
* added a serial proxy for tbb task group and moved the tbb task group
  behind the threading interface.
* added comments to the communicator to better document each method

Plus: coding guidelines compliance.

- add double buffering of spikes and events to communicator
- communicator::exchange() generates buffered events from buffered spikes
- this allows communication alongside cell update methods accessing spike
  and event lists

* coding convention adherence
* cell_local_index_type -> cell_lid_type
* time-type parameterized spikes and events
* communicator reset() clears events in flight
* communicator uses routine for gid->group index mapping
* event_queue 'Event' types to follow specified convention/concept.

Turns out this helps when trying to deliver spikes.

* Makes model.hpp part of library interface; genericize
  value, time types.
* Clarify separation of concerns between model and miniapp.
* Model keeps a record of (local) probe ids for sampler
  attachment by caller.