Derivationtree

Changed

- `DerivationTree.from_node` infers optional `is_open` parameter from whether the given
value is a nonterminal or not.

Changed

- Changed signature of `DerivationTree.children` method, which now returns a dictionary
of paths and nodes instead of only nodes.

Added

- Added convenience methods `DerivationTree.from_node` and `DerivationTree.add_children`
for constructing a derivation tree from a value, optional id, and "open" flag, and
for adding children to an existing derivation tree leaf.

Changed

- Preventing replacement of paths that do not have an immediate parent in the tree.
One *can* replace a path that is a child of a previous leaf node in the tree, but
did not exist in the tree before.