[gfs] Rework evaluator for multivar gf
[gfs] Add no_cast for the mesh_point evaluation.

- fix on_mesh :
   method can be called with mesh_point
   on_mesh wrapper class can be called with lazy arguments.
add make_regular

- make_regular (x) -> return a regular copy, forcing the evaluation
  of expression templates.
- TODO : do it for array_expr, gf_expr
[gfs] mesh product: index_t as a class

- to permit construction from a tuple of mesh points to components
[gfs] Opt -> Evaluator

- Opt template param is replace by Evaluator
- Evaluator : default is void, (default evaluator as before)
- allows to add a custom evaluator (cf new test sym3).
[gfs] Change evaluator concept

- Now evaluator must have one constructor taking (G* g)
  with the gf, given at construction.
- Allow to prepare some data.
- E.g. sym3
[gfs] Test mpi for multivar gf.

TODO : more test.