Add Javadoc for Term.copy to prevent 'unused parameter' warning

Change-Id: I21543ab03ecb0fa2295f549555404bc0aa9f9c61

src/lang/Term.java

diff --git a/src/lang/Term.java b/src/lang/Term.java
index d68e1b5..76cd75f 100644
--- a/src/lang/Term.java
+++ b/src/lang/Term.java
@@ -55,7 +55,11 @@
      */
     public boolean convertible(Class<?> type) { return convertible(getClass(), type); }
 
-    /** Returns a copy of this object. */
+    /**
+     * Returns a copy of this object.
+     * @param engine the engine.
+     *
+     */
     protected Term copy(Prolog engine) { return this; }
 
     /** Returns the dereference value of this term. */