Item for lazy expressions does not work as intended

[lshaw8317]

Expected behaviour for eg.

lexpr = blosc2.lazyexpr('mean(a + b)')
lexpr[:10]

is to slice a[:10] and b[:10] and then calculate mean but this isn't happening.

[lshaw8317]

Relatedly, it would be good if default behaviour of reductions acting on blosc2 arrays were to return blosc2 arrays (not numpy arrays, as happens currently).