Support multiple items in DataTree.__getitem__ and improve NodePath (renamed to TreePath)

[shoyer]

This PR adds support for indexing with multiple items as a list of paths in DataTree.__getitem__, e.g., tree[['first', 'second']].

It also includes internal improvements to NodePath (now renamed to TreePath):

• Rename NodePath to TreePath to make its name slightly more obvious
• Automatically normalize paths in the TreePath constructor
• Use joinpath() and normalized tree paths to simplify implementations of _get_item and _set_item.

[keewis]

looks like there was a similar attempt in #10400, in case it helps

According to our policy, we can drop python=3.11 from 2026-04-04 onwards – you can simulate this by passing today to minimum_versions:

python minimum_versions.py --policy ci/policy.yaml --today 2026-04-04 ci/requirements/min-all-deps.yml

[shoyer]

This is ready for review.

The main thing this could use is clear documentation, to explain that in the case of indexing multiple keys, the resulting DataTree is always defined relative to the node being indexed. This is rather different from the API proposed in #10400, which tries to index the selected variables at each node.

Ideally we could supply this functionality in a dedicated method (which would also make it easier to document), e.g., DataTree.subset() as we discussed last week at the Xarray meeting. This could be similar to the existing discussion about adding a public API for Dataset._copy_listed(): #3894

cc @eni-awowale

[eni-awowale]

Would this be the distinction between the future .subset method and [['a']] selection?

For example with a datatree like:

dt1 = xr.DataTree.from_dict({'/': xr.Dataset(coords={'x': [1, 2, 3]}), '/a': xr.Dataset({'n': 1}), '/a/b': xr.Dataset({'foo': 1})})
<xarray.DataTree>
Group: /
│   Dimensions:  (x: 3)
│   Coordinates:
│     * x        (x) int64 24B 1 2 3
└── Group: /a
    │   Dimensions:  ()
    │   Data variables:
    │       n        int64 8B 1
    └── Group: /a/b
            Dimensions:  ()
            Data variables:
                foo      int64 8B 1

When we select with dt1[['a/b']] we get a datatree that has an empty "a" group, with coordinates from the root group so:

<xarray.DataTree>
Group: /
└── Group: /a
    └── Group: /a/b
            Dimensions:  (x: 3)
            Coordinates:
              * x        (x) int64 24B 1 2 3
            Data variables:
                foo      int64 8B 1

So .subset would do something like dt1.children['a'][['/a/b']], we get a datatree that only returns the b group with the "x" coordinates from root group, so:

<xarray.DataTree 'a'>
Group: /
└── Group: /b
        Dimensions:  (x: 3)
        Coordinates:
          * x        (x) int64 24B 1 2 3
        Data variables:
            foo      int64 8B 1