Build empty polyhedrons set when given equal min/max boundaries.

Also explained better in the javadoc about some wrong usage of
PolyhedronsSet constructor.

JIRA: MATH-1115

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1590254 13f79535-47bb-0310-9956-ffa450edef68