Added test for Luther step interpolator.

BEWARE! This test does not work yet. It confirms there is a problem
in the step interpolator for Luther integrator.

src/test/java/org/apache/commons/math4/ode/nonstiff/LutherFieldStepInterpolatorTest.java

@@ -0,0 +1,44 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.commons.math4.ode.nonstiff;
+
+
+import org.apache.commons.math4.Field;
+import org.apache.commons.math4.RealFieldElement;
+import org.apache.commons.math4.ode.FieldEquationsMapper;
+import org.apache.commons.math4.util.Decimal64Field;
+import org.junit.Test;
+
+public class LutherFieldStepInterpolatorTest extends AbstractRungeKuttaFieldStepInterpolatorTest {
+
+    protected <T extends RealFieldElement<T>> RungeKuttaFieldStepInterpolator<T>
+    createInterpolator(Field<T> field, boolean forward, FieldEquationsMapper<T> mapper) {
+        return new LutherFieldStepInterpolator<T>(field, forward, mapper);
+    }
+
+    @Test
+    public void interpolationAtBounds() {
+        doInterpolationAtBounds(Decimal64Field.getInstance(), 1.0e-15);
+    }
+
+    @Test
+    public void interpolationInside() {
+        doInterpolationInside(Decimal64Field.getInstance(), 3.3e-14, 7.9e-13);
+    }
+
+}