API for reorganizing levels of a datatree

[TomNicholas]

What is your issue?

(Copied from xarray-contrib/datatree#186 - there are a few more comments there)

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@jbusecke and I were discussing API for reorganizing levels of a tree.

For example, say I have 2 models, each which ran 2 scenarios. Thats 4 data-containing leaves in my tree, but there are 2 equally-valid ways to organise this, either model-first or scenario-first.

The model-first tree has node paths:
/mod1/scen1
/mod2/scen1
/mod1/scen2
/mod2/scen2
whilst the scenario-first tree has node paths:
/scen1/mod1
/scen2/mod1
/scen1/mod2
/scen2/mod2

Either of these is equally valid, and one might be preferred sometimes over the other, so we should have a method than can rearrange one structure into the other.

The question is what the API to do this should look like so that it's general, intuitive, and powerful.