Rewrote completely the enclosing spherical cap computation on 2-sphere.

The previous version was based on Welzl algorithm, but it appeared this
algorithm really need some properties that hold in Euclidean spaces and
not on the sphere.

The current version is not perfect in the sense that some times the
enclosing spherical cap returned is not the smallest possible. It is
documented in the Javadoc.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1567599 13f79535-47bb-0310-9956-ffa450edef68