These publications are based upon work supported by the National Science Foundation under Grant No. REC-9804930, REC-0087771, REC-0087573, REC-0228515, REC-0337710, REC-0337710, and REC-0437861 . Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.