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- Implement discrete time systems using events.
- Since systems with discontinuities should have been continuous-time
anyway, the ``dt`` argument to the numeric ``SwitchedSystem`` constructor
was removed.
- The argument rules for ``event_equation_function`` and
``update_equation_function`` during integration now follow
``output_equation_function`` and ``state_equation_function``,
respectively.
- No longer allow non-zero t0 for simulate;