Multidimensional index lists ("index views")

[Ivorforce]

While a bit niche in the tensor space, index views are an undeniably useful feature.

xtensor has support for index views, though returning a copy thereof would run into similar limitations as #8.

Still, selection by index arrays could be very useful, even without a view returned.

[Ivorforce]

8 is implemented. However, as expected, the current implementation does not allow for index views, as the final compute arrays are xarray_adaptor objects. These only support shapes and strides. If we wanted index views, we could add a whole slew of compute arrays, but that would probably explode the binary size.

For now, we should just return a copy, like numpy. We can reconsider later if there is interest.

[Ivorforce]

Implemented in d18ad545bd5aa82f0ec463af88df294221e0897b.