Remove generic event interface on cell_group

`cell_group` had a template method `cell_group::enqueue_events()`, which was parameterized on the type of container used to pass the set of events to enqueue. This PR removes the template, and makes `time_type` a globally defined type in `common_types.hpp`.

The `time_type` that permeated the code is taken from `spike`, which is itself a specialized type alias of `basic_spike`. This is not an intuitive location to define the `time_type`, and hides the fact that as implemented it was effectively a global typedef.

  * Define the default time type in `common_types.hpp`: `using time_type = float`.
  * Use this global `time_type` in the definition of `spike` and `postsynaptic_spike_event`.
  * Replace generic `cell_group::enqueue_events` method with concrete `cell_group::enque_events(const std::vector<post_synaptic_event>&)`.