Introducing a new iterator class which is used by external_array and internal_array.
test_ppq_arrays_and_iterator now tests internal and external arrays as well as the
new iterator and multiway_merge compatibility.
Merge branch 'iterator_adapter' of https://github.com/thk1/stxxl into iterator_adapter

Conflicts:
	include/stxxl/bits/common/stl_iterator_adaptor.h