get_location_at() fixes and unit test

* Fixed a comment with an improper pluralization of one of the class names.

* Fixed the code for get_location_at() as it wasn't handling the case when the time exactly matched an existing point.

* Got rid of an unneeded iterator variable (i) by switching to "for point in self.points:"

* Switched to the 'inbetween' operator style for checking that time falls in the bounds of the segment.