Added test for ppq_external_array.h
Moved internal_array class into the ppq_local namespace.
Moved minima_tree class into parallel_priority_queue.h / ppq_local namespace
* Renamed ppq_external_array.h to external_array.h.

* Reorganized imports in external_array.h
* Moved containers/ppq_index_winner_tree.h to algo/winner_tree.h

* Fixed dummy_custom_stats_counter

* Disabled statistics