- improve the mem_block and shared_block.
- the reference counting is now done in the mem_block and shared_block, removing the need of shared_ptr.
- speed tests shows that shared_ptr is very slow (due to thread safety?)
the new version is much better, though not perfect.
- Hence introducing weak views.
- also :
-- clean the guard mechanism for python (to allow returning from python without any python ref left).
-- clean code, add documentation for mem_block
-- remove nan init, which was not working, and corresponding test
-- serialisation of view still unchanged (need to forbid serialization of view ??).
- tests ok, incl. valgrind tests.
