Using mpl in a more sophisticated manner to pick value_accessor class.

This change has made the code both more concise and more suggestive of its
purpose.


[SVN r19698]

libs/multi_array

@@ -1 +1 @@
-Subproject commit b27fbb3a787e0bd6f591d74396906c9f7fea4721
+Subproject commit 0de9a4b475c9a64cafc66193635bc4288728cde2