row major layout support

[bd4]

Follow convention of xtensor, i.e. add another template param for gt::layout_type::row_major, gt::layout_type::column_major. For backward compatibility we can make the default column_major, but since it's a C++ lib, maybe we should consider switching the default.

See https://xtensor.readthedocs.io/en/latest/container.html?highlight=layout#internal-memory-layout