Added an order 6 fixed-step ODE integrator.

The integrator was designed by H. A. Luther in 1968. We have added a
corresponding step interpolator by solving the order conditions provided
by the rkcheck tool.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1588753 13f79535-47bb-0310-9956-ffa450edef68
Math.nextUp is not available before Java 1.6.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1588754 13f79535-47bb-0310-9956-ffa450edef68