Added function "retention" (previously "perturb" and created documentation; updated documentation; renamed data sets

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/qca@38 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
no comment

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/qca@39 edb9625f-4e0d-4859-8d74-9fd3b1da38cb