[MATH-826] Added SobolSequenceGenerator.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1481558 13f79535-47bb-0310-9956-ffa450edef68
Remove unused import.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1481565 13f79535-47bb-0310-9956-ffa450edef68
Improve gray code calculation.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1481590 13f79535-47bb-0310-9956-ffa450edef68