[gfs,arrays] Fix index generator flat index + 1 bug in matrix_proxy

- gfs : when constructing the index, the i_flat index was not
  recomputed.
- specialize the evaluator for mesh_point: simpler, better perf. no need
  to take modulo
- [arrays] : in the special case of a proxy = same type,
  rare, e.g. g(x) << h(x), which is not usual,
  the operator = (RHS) is not taken for the class itself.
  Usual C++ pb, one needs to overload for default copy assigment.