AthContainersInterfaces: Update clear to work on AuxVectorData.

Update the clear method of IAuxTypeVectorFactory / AuxTypeRegistry to take
AuxVectorData instances rather than bare pointers.  Remove the clear static
method from AuxTypeVector.
Also alter clear to operate on a range, rather than a single element.

Refactoring to make possible dependencies between auxiliary variables,
as would be used to support packed links / jagged vectors.

Control/AthContainersInterfaces/AthContainersInterfaces/IAuxTypeVectorFactory.h

@@ -136,11 +136,15 @@ public:
 
 
   /**
-   * @brief Clear an element within a vector (static method).
-   * @param dst Pointer to the start of the vector's data.
-   * @param dst_index Index of the element in the vector.
+   * @brief Clear a range of elements within a vector.
+   * @param auxid The aux data item being operated on.
+   * @param dst Container holding the element
+   * @param dst_index Index of the first element in the vector.
+   * @param n Number of elements to clear.
    */
-  virtual void clear (void* dst, size_t dst_index) const = 0;
+  virtual void clear (SG::auxid_t auxid,
+                      AuxVectorData& dst, size_t dst_index,
+                      size_t n) const = 0;
 
 
   /**