suppressed a warning for unused constructor

the constructor is called only by externalization

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@919856 13f79535-47bb-0310-9956-ffa450edef68

src/main/java/org/apache/commons/math/ode/jacobians/FirstOrderIntegratorWithJacobians.java

@@ -599,6 +599,7 @@ public class FirstOrderIntegratorWithJacobians {
         /** Simple constructor.
          * <p>This constructor is used only for externalization. It does nothing.</p>
          */
+        @SuppressWarnings("unused")
         public StepInterpolatorWrapper() {
         }