Derivationtree

Changed

- `DerivationTree.paths`,`DerivationTree.leaves`, and `DerivationTree.open_leaves` now
return generators instead of whole dictionaries for better performance.

Changed

- Fixed `DerivationTree.is_valid_path`
- Path objects are memoized
- Potential performance improvement in `DerivationTree.replace_subtree`
- Performance improvement in `DerivationTree.__init__`

Changed

- Implemented `PathIterator.__getitem__` to enable, e.g., unpacking of Paths

Changed

- Using new `Path` objects instead of tuples for paths. This avoids the necessity to
convert between datrie string keys and numeric paths, the cost of which accumulates
over time.
- More efficient `DerivationTree.replace_trie`
- More efficient `DerivationTree.leaves`
- More efficient `DerivationTree.find_node`
- Fixed `DerivationTree.structural_hash`
- Fixed `DerivationTree.is_potential_prefix`

Changed

- Fix in `DerivationTree.to_string` method; now, not displaying (closed) nonterminal
leaves
- Don't use deep copy in `DerivationTree.replace_path`, but create new datrie object.
This is obviously more efficient. However, there should be a more (time & memory)
efficient way of replacing a leaf in a tree than copying the whole datrie...
- More efficient `DerivationTree.__eq__` and `DerivationTree.is_leaf` methods
- `DerivationTree.paths` can return mappings from datrie keys to nodes if requested

Changed

- Made method `DerivationTree.trie_from_init_map` private (`__` prefix)
- Method `DerivationTree.is_complete` now returns True iff the whole *tree*, and not
only the root (or otherwise specified) node is not open.
- Method `DerivationTree.filter` now returns a dict instead of a list of pairs.
- Bug fix in `DerivationTree.replace_path` for trees with a non-trivial root path.