make sure the abstract copy() method signature used is the restricted one

inherited from the StorelessUnivariateStatistic interface and not the
general one inherited from AbstractUnivariateStatistic.
The eclipse compiler generates an error without this.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@723694 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/commons/math/stat/descriptive/AbstractStorelessUnivariateStatistic.java

@@ -91,6 +91,11 @@ public abstract class AbstractStorelessUnivariateStatistic
         return getResult();
     }
 
+    /**
+     * {@inheritDoc}
+     */
+    public abstract StorelessUnivariateStatistic copy();
+
     /**
      * {@inheritDoc}
      */