Added a getEnclosingCap method for spherical polygons.

In simple cases (small polygon, one piece only, no holes), the enclosing
cap will be the smallest one, but this is not guaranteed in the general
case.

The enclosing cap can be used to speed up inside/outside points when the
region is small with respect to the sphere as with only one check of an
angular separation, most points of the sphere can be safely identified
as outside. The more time-consuming checks involving the full boundary
are therefore done only for a small portion of the sphere.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1566358 13f79535-47bb-0310-9956-ffa450edef68