This work was partially supported by the Netherlands Organization for Scientific Research (NWO) Grant 900-538-040, the Foundation for Technical Sciences (STW) Project 2987, the ASCI PostDoc program, and the Rolling Grants program of the Foundation for Fundamental Research in Matter (FOM). Images presented above were processed using TCL-Image and SCIL-Image (both from the TNO-TPD, Stieltjesweg 1, Delft, The Netherlands) and Adobe Photoshop&tm;.