generalize time sequences (#496)

Changes to libarbor
-------------------------

Time sequences were added in `src/time_sequence.hpp`:
- added new `time_seq` type that implements a type-erasure interface for the
  concept of a time sequence generator.
- added poisson, regular and vector-backed implementations of the time sequence
  concept.

Event generators:
- The poisson, regular and vector-backed implementations of the event generator
  concept were refactored to use the.

Cell groups:
- Removed the `dss_cell_group` and `rss_cell_group` and associated types.
- Added a generic spike source cell  that generates a sequence of spikes
  at time points specified by a `time_seq`. Using this approach, an
  additional `cell_group` specialization is not required for each type of
  sequence, and user-defined sequences can be used with minimal overhead.

Unit tests
------------

- Added unit tests for `time_seq`.
- Simplified `event_generator` unit tests, because much of the testing
  of the sequences was moved to the `time_seq` tests.
- Added unit tests for `spike_source_cell_group`.

Changes to miniapp
-------------------------

- simplified the miniapp by removing the command line options for using an input spike chain from file.
- updated the miniapp recipe to use `spike_source` cell group instead of `dss_cell_group`.