[BUG] non-uniform rebinning does not work with Weight() storage

[Saransh-cpp]

Steps to reproduce

import boost_histogram as bh

hist = bh.Histogram(bh.axis.Regular(10, 0.0, 1.0), storage=bh.storage.Weight())
hist.fill(0.9)

rebin = bh.rebin(groups=[1,2,3,4])
hist[::rebin].view()

would result in -

UFuncTypeError: ufunc 'add' did not contain a loop with signature matching types (dtype([('value', '<f8'), ('variance', '<f8')]), dtype([('value', '<f8'), ('variance', '<f8')])) -> None

Working on it!

Also see scikit-hep/hist#345 (comment)