Binary sensor for detecting linear trends (#9808)

* Trend sensor now uses linear regression to calculate trend

* Added numpy to trend sensor test requirements

* Added trendline tests

* Trend sensor now has max_samples attribute

* Trend sensor uses utcnow from HA utils

* Trend sensor now completes setup in async_added_to_hass

* Fixed linter issues

* Fixed broken import

* Trend tests make use of max_samples

* Added @asyncio.coroutine decorator to trend update callback

* Update trend.py