SPARK-4022 [CORE] [MLLIB] [WIP] Replace colt dependency (LGPL) with commons-math

This change replaces usages of colt with commons-math3 equivalents, and makes some minor necessary adjustments to related code and tests to match.
It should still be one-sided. The argument to the Colt method (http://acs.lbl.gov/ACSSoftware/colt/api/cern/jet/stat/Probability.html#studentTInverse(double, int)) was `alpha` such that the cumulative probability is `1-alpha/2`. For the Commons Math3 method, the argument is just the cumulative probability (http://commons.apache.org/proper/commons-math/apidocs/org/apache/commons/math3/distribution/AbstractRealDistribution.html#inverseCumulativeProbability(double)). So I translated as `1 - (1 - confidence) / 2`.