cleanup and updates

* moved static prototypes in an extern header (++readability)
* added a string for each son of each node (attribute_value)
* changed the way to display the tree :
(attribute) has become -attribute
[leaf] has become .leaf
* some functions have been added (such as get_tree_size,,
leave_memory_error, etc.)
* added possibility to load a tree which has been saved (displayed) in a
file. The extension is NEL for Nodes, Edges and Leaves.
* updated TODO list