Support adding a new sub-column via `NestedFrame.reduce()`

[hombit]

Feature request

It would be great, if .reduce() could add a new sub-column directly to existing nested column. Example with a toy UDF:

from nested_pandas.datasets import generate_dataset

nf = generate_dataset(5, 4)
nf.reduce(lambda flux: {"nested.x": flux - flux.mean()}, "nested.flux", append_columns=True)

Currently, this would produce the second "nested" column, while it would be really nice if it would append a new sub-column directly to exiting nested column.

My current workaround is

new = nf.reduce(lambda flux: {"new.x": flux - flux.mean()}, "nested.flux")
nf["nested.x"] = new["new.x"]

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Please check the following:

• I have described the purpose of the suggested change, specifying what I need the enhancement to accomplish, i.e. what problem it solves.
• I have included any relevant links, screenshots, environment information, and data relevant to implementing the requested feature, as well as pseudocode for how I want to access the new functionality.
• If I have ideas for how the new feature could be implemented, I have provided explanations and/or pseudocode and/or task lists for the steps.

[mi-dai]

I was trying to do something similar and used the workaround @hombit posted. Would be great to have this feature to directly assign nested columns for reduce. Please also add more documentation about this (For example, I didn't know the user-defined function should be returning a dictionary.)