Add accessors

[TomNicholas]

• Closes Extend datatree with custom accessors #143
• Tests added
• Passes pre-commit run --all-files
• New functions/methods are listed in api.rst
• Changes are summarized in docs/source/whats-new.rst

[TomNicholas]

            @register_datatree_accessor("demo")
>           class Foo:
E           Failed: DID NOT WARN. No warnings of type (<class 'Warning'>,) were emitted. The list of emitted warnings is: [].

datatree/tests/test_extensions.py:39: Failed

This error occurs both locally and in CI, which is weird, because I just copied the code across from xarray.core.extensions and xarray.tests.test_extensions.py 😕

[keewis]

that's not weird, actually: the xarray test checks both Dataset and DataArray accessors at once, and thus it can delete the Dataset accessor and later check that redefining the DataArray accessor does correctly emit a warning (which DataTree can't, of course, because we're testing just a single class).

To fix this test, you can just move the del check below the warning check.

[TomNicholas]

that's not weird, actually

Good point, well made!