Add duration parameter to DataStream run method

[hnewey7]

Added a duration parameter to the DataStream.run() method, allowing users to input a timedelta object to specify how long the DataStream should run for.

To implement this I've created a separate method called _run which takes the timedelta object and converts it into seconds. An aynscio task is then created using asyncio.create_task(self._run_forever()) and then this task is waited for using asyncio.wait(). The asyncio wait function has a timeout parameter in seconds which in this case is used to cancel the running of _run_forever when the duration is exceeded.

I've also added a unit test with a number of durations to test this feature.