mpi for the gf

- done Matsubara freq for testing and rereading.
- TODO: generalize to other meshes.
- draft for multi var gf
[gf] Multiple var. Add default target

- change the underlying data : do not flatten the linear indices of the
  mesh into a single index, keep a higher dim array.
- easier for various places, and necessary for g(nu,nu').
- work on several 2 part. containers.
- add default target (may not be always matrix_valued)
save
draft m_tail