Expose dataset reduce operations

[TomNicholas]

A WIP attempt to get operations like * and + working on DatasetNode objects.

Idea is to copy the pattern used to add these operators to xarray.Dataset, where Dataset inherits from DatasetArithmetic.

DatasetArithmetic itself inherits from about 5 different Mixin classes, which I don't particularly want to regenerate, especially as DatasetOpsMixin is generated by templating in xarray. Here we instead just want to take the methods defined on DatasetArithmetic and decorate them with map_over_subtree, to get a DataTreeArithmetic object to inherit from.

Problem is at the moment I'm getting errors like this:

> result = dt * dt
E TypeError: unsupported operand type(s) for *: 'DatasetNode' and 'DatasetNode'

I don't understand why this is happening because __mul__ and __rmul__ are definitely defined on the dt objects. (Could it be because those methods get defined only once the object instance has been created, and not when the class is defined?)

[TomNicholas]

In a twist of events this PR has ended up accidentally successfully implementing reduce and cumulative methods such as .mean() and .cumsum(). This happened because I generalized the PR to make DataTree inherit from xarray.core.Dataset.DataWithCoords and xarray.core.Dataset.DatasetArithmetic instead of just xarray.core._typed_ops.DatasetOpsMixin (which is a superclass of xarray.core.Dataset.DatasetArithmetic).

However I still haven't got numpy ufuncs or binary arithmetic operations like multiply working, but I'll discuss them in separate issue.