minor percentile related updates

* corrected removeAbovePercentile description
* corrected removeAboveValue description
* corrected removeBelowPercentile description
* corrected removeBelowValue description
* corrected removeBelowPercentile function mapping
* nPercentile perc_val test corrected
  * should return results, even if they are empty
* nPercentile name updated--it should match usage
Merge pull request #127 from timzehta/functions