Full UHI

[henryiii]

The full UHI includes arbitrary reducers and rebinnings. This will need to be implemented before 1.0, but depends on #194 and #189; those should go into 0.6.0 so let's nominally target 0.7.0 for a full UHI implementation.

[henryiii]

Suggested API (rebinning):

class Rebinner:
    def __call__(self, axis: PlottableAxis) -> Sequence[int] | Mapping[int | str, Sequence[int | str]]:
        """
        axis: axis object
        """

On Regular, Variable, Integer axes, this would return a sequence. For example, Integer(0, 10), [0,5,2,3,0] (probably the output of bh.rebin([5,2,3])) would become Variable([0,5,7,10]). First and last groups get put into underflow/overflow (or removed, if no flow bins). Summing this should always equal the total number of bins, error if not. For Cat axes, mappings are used. So for IntCat([1,3,5,7,9]), then a valid rebin would be {0: [1,3], 1: [5,7,9]}}. Another might be {"13": [1,3], "579": [5,7,9]}}.

@jpivarski, I know that reducers were also planned. I think a reducer can't be single argument (It at least needs the full data + the axis it's operating on?), so I think this won't block future implementation of that if needed?

For bh.rebin, we could come up with a nice API for common operations: edge based rebins, group-based rebins (making flow parameters, like bh.rebin([1,2], overflow=3)), and mappings.

[eduardo-rodrigues]

I confess I do not have in my head all of UHI but I have the feeling that this could be closed at this point, @henryiii.