Un-deprecated RealVector.sparseIterator.

The documentation now explicitly states that entries not iterated above
are the zero ones.

This is part of Apache Commons Math reconsidering support for sparse
linear algebra.

JIRA: MATH-875

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1570510 13f79535-47bb-0310-9956-ffa450edef68