Change private least square problem implementation to internal class.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1571014 13f79535-47bb-0310-9956-ffa450edef68
Moved OptimizationProblem interface to optim package.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1571015 13f79535-47bb-0310-9956-ffa450edef68