Specialize get_strides_type for xbuffer_adaptor

[AntoinePrv]

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Description

Fix xtensor-stack/xtensor-r#85
Solution discussed with @JohanMabille

[AntoinePrv]

So it turns out, after compilation succeed, there is a second issue.

When the shape (inner_shape) is copied, it may be an xbuffer_adaptor, so the follwowing lines actually modify the shape.

xtensor/include/xtensor/xmath.hpp

Line 2111 in 275a4ef

auto keep_dim_shape = e.shape();