Fix issue #224 test assertion failure. (#225)

`cell_tree::depth_from_root()` was incorrectly traversing only
the first branch from the tree root, leading to uninitialized
values in the returned depth array.

This leads to an incorrect maximum leaf node in `find_minimum_root()`,
which then returns `no_parent`. This gets passed to
`tree::change_root(size_t)`.

* Correct `cell_tree::depth_from_root()` implementation.
* Re-enable curiously disabled test case in `cell_tree.from_parent_index`
* Add unit tests for `cell_tree::depth_from_root`.