Fixed sphere generation in degenerated cases.

In almost coplanar / almost colinear cases, sphere generation could
diverge in such a way the support points did not belong to the sphere
anymore! This new implementation uses exact arithmetic for the
computation. It is much slower, but since very few balls should be
generated during the Welzl algorithm execution, this is probably
acceptable.

JIRA: MATH-1096

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1563712 13f79535-47bb-0310-9956-ffa450edef68