keep special attributes of step siaf when parsing it in twinstim()

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/surveillance@715 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
used resaveRdaFiles() to reduce file size

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/surveillance@716 edb9625f-4e0d-4859-8d74-9fd3b1da38cb