Remove dependency on anytree

[TomNicholas]

The code currently has a hard dependency on the anytree library, which it uses primarily to implement the actual .parent & .children structure of the TreeNode class, through inheritance from anytree.NodeMixin.

We don't need this dependency - anytree is not a big project, and we can simply reimplement the classes we need within this project instead. We should give due credit in the code because anytree has an Apache 2.0 license (plus Scout's honor).

Reimplementing anytree's functionality would also allow us to change various parts of it. For example we don't need support for python 2, we can standardize the error types (e.g. to return KeyError instead of e.g. anytree.ResolverError), and get rid of the mis-spelled .anchestors property. We shouldn't need to worry about breaking the code because the tree functionality is already covered by the unit tests in test_treenode.py.

[TomNicholas]

I think that for the __slots__ defined in DatasetArithmetic and other places in xarray to work then the whole chain of subclasses must define slots, but anytree.NodeMixin does not define it. Unless we can monkey-patch __slots__ onto anytree.NodeMixin then this would be another benefit of re-implementing NodeMixin ourselves.

[TomNicholas]

Closed by #76