- Can now retrieve or set a column/row of a tensor.
  Either compile-time or run-time checks.

  Get
     t.col<1>();   t.col(1);
     t.row<1>();   t.row(1);

  Set
     t.col<1>(vec);   t.col(1,vec);
     t.row<1>(vec);   t.row(1,vec);

  The templated versions are compile-time checked

     t.col<3>();
     t.col<3>(vec);

  The parameter versions are run-time checked

     t.col(3);
     t.col(3,vec);

ENH: provide named access to tensor/tensor inner product as inner()