ENH: define higher order events

- define_events function for creating new events via time lag between exisiting events
- example: plot fast motor responses
COSMITS + ENH: adding test, fixing doc string
ENH: fixing integer issue + improving example
COSMITS
COSMITS: updating doc text.
ENH + COSMITS: addressing points from review, improving example

- adding subplot
- changing name to define_target_events
- adding test doc string
- returning empty array instead of None
- moving function to event name space
COSMITS: units
cosmit