enable navigation tree

Author: eddelbuettel

doxyfile

@@ -1048,7 +1048,7 @@ ENUM_VALUES_PER_LINE   = 4
 # JavaScript, DHTML, CSS and frames is required (i.e. any modern browser). 
 # Windows users are probably better off using the HTML help feature.
 
-GENERATE_TREEVIEW      = NO
+GENERATE_TREEVIEW      = YES
 
 # By enabling USE_INLINE_TREES, doxygen will generate the Groups, Directories, 
 # and Class Hierarchy pages using a tree view instead of an ordered list.