This article is published under a Creative Commons Attribution License by Elsevier and available electronically from http://www.sciencedirect.com/science/article/pii/037026939500363P.
Except where otherwise noted, this item's license is described as: This article is published under a Creative Commons Attribution License by Elsevier and available electronically from http://www.sciencedirect.com/science/article/pii/037026939500363P.