Fix xview data offsets with multiple leading newaxis slices

[vera-rubin]

Checklist

• The title and commit message(s) are descriptive.
• Small commits made to fix your PR have been squashed to avoid history pollution.
• Tests have been added for new features or bug fixes.
• API of new functions and classes are documented. (No API change.)

Description

As documented, newaxis() adds a dimension of length one, so this view should select the second row:

xt::xarray<int> a = {{0, 1}, {2, 3}};
auto v = xt::view(a, xt::newaxis(), xt::newaxis(), xt::range(1, 2), xt::all());
xt::xarray<int> copied = v;
v = 100;

Direct indexing returns {2,3}, but copying produces {1,2} and assignment leaves a as {{0,100},{100,3}} instead of {{0,1},{100,100}}.

We compute each slice offset using its original axis stride, accounting for preceding new axes. We added a regression covering the offset, copying and scalar assignment, and ran the existing xview and xview semantic tests.

— ChipLab