Fix norm interface to match xtensor

[wolfv]

As per xtensor-stack/xtensor#417

[wolfv]

This might require some discussion.

Should we specialize the norms for certain types so that they are handled by BLAS vs. xtensor?
Or should blas norms just live in the linalg namespace while the xtensor norms are in the xt namespace?

Also, should we extend the norms in xtensor-blas to take the axis parameter? Norms in xtensor-blas would also be greedy by default ...