Make sure that cycle_start state is set before simulating steps.
Simplify setting of STATE_CYCLE and ISR interval.

Set sys.state to STATE_CYCLE directly instead of calling back to
st_wakeup().

Convert get_step_time() to a constant and rename it to ISR_INTERVAL.
Merge pull request #337 from michmerr/edge

Make sure that cycle_start state is set before simulating steps.