Fix helper class used in StdDev test, tighten error margins and add tests.
Fix helper class used in StdDev test, tighten error margins and add tests.

Signed-off-by: Benoit Sigoure <d0ba46810740e35622837d76cd77ceb8c4bf46f5@gmail.com>

andyflury/opentsdb,sidhhu/opentsdb,johann8384/opentsdb,MadDogTechnology/opentsdb,eswdd/opentsdb,johann8384/opentsdb,MadDogTechnology/opentsdb,ShefronYudy/opentsdb,sidhhu/opentsdb,andyflury/opentsdb,alienth/opentsdb,ShefronYudy/opentsdb,OpenTSDB/opentsdb,eswdd/opentsdb,OpenTSDB/opentsdb,alienth/opentsdb,MadDogTechnology/opentsdb,OpenTSDB/opentsdb