transform to ostream_iterator fails

[jaredhoberock]

Reported here:

https://groups.google.com/forum/?hl=en&fromgroups=#!topic/thrust-users/RBjsMXImDxI

[jaredhoberock]

One way to fix this might be to create a new type output_iterator_adaptor or somesuch which would provide a dummy reference typedef for output iterators who don't have one (or whose reference type is void, like std::ostream_iterator).

[jaredhoberock]

We could properly fix this with c++11. The idea would be for zip_iterator::operator*() to return a tuple of whatever the constituent iterators return, instead of tuple<iterator_reference<Iterator>::type...>.

The idea would be to first check to see if iterator_reference<Iterator> is reasonable. If it is, return it. If not (e.g., if it is void or does not exist), use the result of Iterator::operator*.