An immediate reduction keeps the reduced axis in shape

[FrancescAlted]

For example, when I type in the prompt:

imag = mean(@public/examples/numbers_color.b2nd, axis=0)

This gives a dataset with shape = (368, 744, 4) as a result, which is correct.

But, if I do (note the walrus operator):

imag2 := mean(@public/examples/numbers_color.b2nd, axis=0)

This outputs a computed datasets with shape = (1, 368, 744, 4) as a result, which is not wrong, but it does not follow numpy conventions.

[lshaw8317]

Fixed in blosc2 (see Blosc/python-blosc2#404).