Reducitons over a subset of axes (functional in PyActiveStorage, needs Reductionist implement)

[davidhassell]

We want to be able to do reductions over a subset of axes, i.e. emulating numpy's the axis keyword argument to is reduction functions (e.g. np.sum(x, axis=(0, 2))

[valeriupredoi]

#248 merged, but let's keep the issue open until we get Reductionist backend support for it 🍺

[davidhassell]

Thanks, V - sounds like plan.