Fix DataTree.from_dict to be insensitive to insertion order

[TomNicholas]

• Closes DataTree.from_dict fails to insert parent if parent/child inserted first. #9276
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

[kmuehlbauer]

@TomNicholas What if the user does want something like the following?

reversed = DataTree.from_dict(
            {
                "/": xr.Dataset({"age": 83}),
                "/C/Chloe": xr.Dataset({"age": 10}),
                "/A/Abe": xr.Dataset({"age": 39}),
            }
        )

[TomNicholas]

Ah hmm good question. Well this PR will succeed, but create a tree with the children in a different order:

<xarray.DataTree>
Group: /
│   Dimensions:  ()
│   Data variables:
│       age      int64 8B 83
├── Group: /A
│   └── Group: /A/Abe
│           Dimensions:  ()
│           Data variables:
│               age      int64 8B 39
└── Group: /C
    └── Group: /C/Chloe
            Dimensions:  ()
            Data variables:
                age      int64 8B 10

Whether or not this is okay depends on whether or not we consider the children to be ordered. Internally they are ordered, which this fix would break I guess. But do we want to guarantee preservation of that order to users? I think it should behave the same way as whether or not we guarantee preservation of the order of .variables in Dataset.

cc @shoyer

[TomNicholas]

@keewis suggested that if we do a sortby depth then we would eliminate the bug without re-ordering the nodes within a group.

[keewis]

basically, we sort by the number of / characters in the (normalized) paths

[TomNicholas]

Fixed now, and tested. What a neat idea @keewis !

[kmuehlbauer]

Wonderful! 💯

[TomNicholas]

This assertion fails unless sorted sorts by depth.

[flamingbear]

I had to think on this, but I like the way it works.

[flamingbear]

This tracks, I like the depth order nice touch.

[TomNicholas]

The test failure is an unrelated flaky dask test so now I have an approval I will merge.