Import Geant4 8.1.0 source tree

source/visualization/XXX/include/tree/Changelog

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+Version 0.5.0, 1/2/2006
+  - Realized that assignment to *i for const_iterators was
+    being allowed.  Fixed.
+  - Forced instantiation of whole template and caught
+    several typos (iterator in place of const_iterator, etc.)
+  - Can now find the size of subtrees using a size() method
+    of iterator or const_iterator
+  - i += n for sibling_iterators
+  - Added range constructor to build n-ary trees
+  - Allow conversion from iterator to const_iterator
+  - Added height() and height_leftmost() tree methods
+  - First serious attempt at pointer specialization.  It is
+    disabled by default because it is incomplete and programs
+    with only a few types of pointer trees will get larger.
+    If you have many different types of trees, then the program
+    size should increase less with specialization enabled.
+    Suggestions on the best way to implement specialization
+    are welcome.
+
+Version 0.2.1, 12/17/2005
+  - Compatible with gcc 4.0
+  - Pruning for const_preorder_iterator
+  - Added depth counter for iterators
+  - Added addition operator to sibling_iterators so that you can
+    do stuff like j = i + 1.
+
+Version 0.2.0, 04/05/2005
+  - Now compatible with gcc 3.4
+  - In between 0.1.0 and 0.2.0, the tree class saw
+    extensive use representing parse trees and various
+    other tree structures in my own programs.  Remaining
+    bugs are likely in methods I don't use very often
+    but provided because they seemed useful.
+
+Version 0.1.0, 07/18/2003
+  - very general tree template
+  - STL compatible