This work is being done within ESPRIT project 6253 ``Supercomputer Highly Parallel System'' SHIPS, funded by the European Union, through DG III of its Commission, HPCN Unit. We deeply appreciate this financial support, without which this work would have not existed.
We would like to thank Catherine Chronaki and Manolis Katevenis for useful comments in earlier drafts of this paper. A. Labrinidis, and A. Zaras provided useful feedback.