Tame `time_type` proliferation. (#203)

Differing classes had their own time_type and other classes were parameterized on it in ways that were compatibile, but only by chance. 

With these changes, modifying the time type used in spike will propagate through to all dependent classes.

  * Rename generic spike<I, T> as basic_spike<I, T>. 
  * Use spike = basic_spike<cell_member_type, float> as the common spike type.
  * Replace instances of spike_type aliases with just spike.
  * time_type aliases are defined in terms of spike::time_type.
  * Remove time_type parameterization in connection.
  * Remove time_type parameterization in communicator.
  * Remove time_type parameterization in exporter classes.