* external_array completely rewritten (preliminary commit in seperate file):
	- can have an arbitrary number of blocks in RAM now
	- provides a write buffer of arbitrary size. multiway_merge can read/write directly from/into the typed_blocks
	- having access to the maximum value of all blocks
	- first block is never written to external memory
	- releasing BIDs right after a block is removed
	- cleaner code

	parallel_priority_queue does not use this class yet!

* new class random_access_iterator_adaptor that provides a std::iterator base class for other iterators
	like std::vector<T>::iterator. Needed for compatibility of external_array and internal_array
	iterators in the multiway merge.
fixing iterator_adaptor
fixing iterator_adaptor