Add the required interface class for QStatistics.java

Adding back the required interface class for QStatistics.java.
It will cause "java.lang.ExceptionInInitializerError" when it is missing.
**This is not an update to the implementation of CDCMS. This file had been used in the experiments of the published paper. I just forgot to upload it. My apologies.**

moa/src/main/java/moa/classifiers/core/diversitytest/DiversityTest.java

@@ -0,0 +1,13 @@
+package moa.classifiers.core.diversitytest;
+
+import com.yahoo.labs.samoa.instances.Instance;
+import java.util.List;
+import java.util.concurrent.Callable;
+
+import moa.classifiers.Classifier;
+import moa.options.OptionHandler;
+
+public interface DiversityTest extends OptionHandler, Callable<Double> {
+	public void set(List<Instance> testChunk, List<Classifier> targetPool);
+	public boolean morePositiveMoreDiverse();
+}