New function "as.px.data.frame" to generate "px-objects" from "data.frame".
Improved functionality for reading and writing files with KEYS (most efficient to storing sparse array).
Fixed some minor bug



git-svn-id: svn://svn.r-forge.r-project.org/svnroot/pxr@100 edb9625f-4e0d-4859-8d74-9fd3b1da38cb