[gfs] Ref count error in set_item in gf_desc

- First build the tuple, then use it and let pyref decref it
  automatically.
- The current construction was not correct.
[arrays, test] Fix one test in TRIQS_ARRAYS_ENFORCE_BOUNDCHECK mode

- When the macro is given, this test must undef it
  we only check a internal part of the lib, no data.
Get rid of <<=, replaced by <<. Add warning

- Replace in tests, doc.
- Add a warning of deprecation for <<= into <<.