gfs: Fix bug in imfreq with <0 freq.

- there was a confusion in gf imfreq, in the new case
where freq can be <0 (non real gf, or for product gf).
- index: is the matsubara n, as in the struct matsubara_freq
  index can be >0 or <0
- linear_index : is the shift from the 0. It is always >0.
  Fixed function to compute it.

- Also changed the construction of mesh_point in the generic iterator.
  Before, was constructed with a mesh point of index 0
  Now, added a new constructor on mesh_point_t, just taking the mesh
  which construct the *first* mesh_point.
  Fixed linear, discrete, product accordingly.
  Added to the documentation of the concepts of gf.