Added
- `child_index` property to `TableauxNode`.
- More extensive tests for `TableauxSystem`'s `is_correct_tree` method.
Changed
- Error messages on `logics.classes.propositional.proof_theories.tableaux`.
`TableauxSystem`'s `is_correct_tree` method now returns a list of tuples
as `error_list`, where the first member is the tuple of indexes (`int`) leading to the
node from the root and the second is an `str` with the error, instead of a list of `str`.