Convenient access to (pre)terminals on Derivation tres

[goodmami]

If one wants to get the lexical entry or type of the leaves of a derivation tree, they must traverse the tree until a daughters list with a single UdfTerminal is found (since the preterminal is where lexical type and entry info is stored). Since this might be a likely usage of derivation trees, a more convenient function could be created to do this. E.g. d.leaf_info() or something, which returns a list of (lexical_entry, lexical_type, orth). Alternatively, d.terminals() and d.preterminals(), which could be more generic.