issue_comments: 873492892

Some other thoughts about tags:

1) Does the definition of tags include variable names of DataArrays? I think it should.

2) As @martinitus mentioned, a DataTree containing only leaves with only 1 tag each is effectively a Dataset. I wonder if Dataset could be refactored to be a special case of a more general DataTree, possibly as a subclass?

3) Selecting via tags would need to allow a distinction between "get me all leaves with these exact tags" and "get me all leaves whose tags include these ones". Maybe dt.choose_only(tags) and dt.choose_all(tags)?

4) The latter type of tag-based access would make plotting different leaves against one another easier too - given a multi-resolution (or multi-model) datatree like this:

dt |-- high_res | |-- temperature | |-- CO2 |-- medium_res | |-- temperature | |-- CO2 |-- low_res | |-- temperature | |-- CO2

then assuming that the definition of tags included the DataArray variable names, then

dt.choose_all('temperature').plot.line(x='time')

would select all leaves with a tempature tag, check that the temperature DataArrays had the same dimensions (but no need for any time coordinates to share size or values), and then plot them against one another on the same axes. This would be so useful - I would say this use case is 90% of the reason users iterate over dictionaries of datasets currently.

5) With a tag-based system you can create cycles of tags, like A&B, B&C, C&A, which you can't really do with hierarchical trees. I don't think that actually causes any problems though...