Layout as template parameter

[JohanMabille]

Containers in xtensor should take an additinoal layout template parameter with three possible values:

• row_major
• column_major
• dynamic_layout

This would facilitate the integration of BLAS libraries and the use of SIMD intrinsics for most common layouts. Doing this, the following items should be taken into account:

• in place transpose should be disabled for row_major / column_major. In order to keep consistency, I think we could remove these functions from the xcontainer interface and add a transpose view. This would break backward compatibility.
• the reshape(const shape_type&, const strides_type&) overload should be disabled for row_major / column_major.

[wolfv]

Sounds good. I'd propose a strided_view.
That'll most likely also work for #161 :)

[JohanMabille]

Implemented in #255.