Fix arithmetic of mesh_points in gf

- the arithmetics of the gf mesh_points :
the forwarding of the second argument was missing,
leading to subtle bugs e.g. in  (in matrix case) :

  deltaw(iom_) << iom_ * gw.singularity()(-1) + gw.singularity()(0) - gw(iom_);

because the partial evaluation of the expression introduced a dangling const matrix_view &
instead of moving the temporary matrix_view into the final matrix expression template.
cleaning clef code. TO MERGE