gf: change to <Variable,Target,Opt>

- split the descriptor into traits.
- update cython
- only change for user is :
  gf<block<XX>> --> gf<block_index, gf<X> >
- make of mesh now in mesh trait
add gf_two times for Keldysh computation

   Rebased & merged from work of Laura, on her branch.

   Author: Laura Messio <laura.messio@cea.fr>
python support for gf 2 times
gf: fix ambiguity in ops for 2 mesh_pt

- m+m, m-m, ...  : disable one version when same rhs, lhs
add new mpi reduction test for gf
add 2times->1time slicing for g keldysh
add template alias blockgf to gv2 example

- work on all compilers, but gcc 4.6