Support slices over multiple dims

[bencottier]

Note: there are different meanings of dims (#18 ), but the fundamental issue is that eachslice only supports single dimensions.

The problem arises in the following example. I want to apply MeanStdScaling, which internally computes mean(data; dims=3) and std(data; dims=3) in the sense of reducing the data on dimension 3, in a 3D array. Currently, to do that, eachslice would need to take dims=[1, 2], but it only supports single dimensions. Therefore we might need to move away from eachslice.

[glennmoy]

no longer applies