gf/clef. Bug fix, and added scalar->matrix reinterpret for blocks

- clef : fix a little bug in storage when evaluating
  (was using the wrong trait to deduce storage type).
- gf : block :
     - added reinterpret_scalar_valued_gf_as_matrix_valued
       for block function
     - cleaned make_block_gf_view_from_vector
     - added make_block_gf_view_from_vectormake_block_gf_view_from_vector_of_cython_proxy
       and changed the cython accordingly because it requires a slightly different syntax.
     - updated tests
- gf : cleaned some template.
gf:fourier Add missing function in no_tail case

- one only the Matsubara case.
- TODO: the real omega case : decide and implement.
- impl: remove superfluous dispatch in x_impl