factor out the _interpolation into it's own function
replace total_seconds for compatability with 2.6

The datetime.timedelta object in Python 2.6 lacks a total_seconds method.
split _to_keep into it's own function
fix test_sixSigma_has_default_arguments by using dependency injection
fix tests, all the real functionality is now in the other methods
Merge branch 'feature/predictive_monitoring' into is24-master

* feature/predictive_monitoring:
  fix tests, all the real functionality is now in the other methods
  fix test_sixSigma_has_default_arguments by using dependency injection
  split _to_keep into it's own function
  replace total_seconds for compatability with 2.6
  factor out the _interpolation into it's own function