topaz::zip_iterator does not return references

[hamsteri15]

The implemented zip_iterator::dereference() always returns a tuple of dereferenced values of the underlying iterators. This makes it unusable for transform like algorithms which is clearly wrong. The issue is in the dref_tuple_elements function which should somehow check if the *it is a rvalue reference (from a transform operation) in which case value should be returned. Otherwise it should return a tuple of lvalue references so that assignment is possible.