Added implementation of PSquare algorithm to estimate percentiles without
storing data in memory (i.e. as StorelessUnivariateStatistic).

JIRA: MATH-418
Contributed by: Venkatesha Murthy



git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1604443 13f79535-47bb-0310-9956-ffa450edef68