change the prototype of s4u::Comm::wait_any to mimick the C code
Returning an iterator was not working as we returned every elements in
the input collection, without any sorting (fix #170).
Returning a collection or an iterator is difficult, as is. We would
have to build the collection in the backstage, iterating over the
provided comms to see which ones are done.
Plus, the user would probably want to then find these comms in the
provided vector to remove them from there.
In the end, that would be a lot of useless and potentially unefficient
code. It seems much better to provide the index in the vector provided
by the user. If she wants, she can iterate from there to see if the
comms are to be removed, or she can just remove this one and fire
another wait_any.
Maybe less C++ish, but more efficient and not that ugly either.
