CxxUtils: Add ConcurrentRangeMap::clear().

Mostly for testing --- should not normally be used.

Control/CxxUtils/CxxUtils/ConcurrentRangeMap.h

@@ -479,6 +479,13 @@ public:
   size_t trim (const std::vector<key_query_type>& keys);
 
 
+  /**
+   * @brief Remove all entries in the container.
+   *        Mostly for testing --- should not normally be used.
+   */
+  void clear();
+
+
   /**
    * @brief Return the current number of elements in the map.
    */