twinstim: also throw more helpful error messages for .siafDeriv


git-svn-id: svn://svn.r-forge.r-project.org/svnroot/surveillance@778 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
avoid codetools::checkUsage note about unknown variable 'x' in curve()


git-svn-id: svn://svn.r-forge.r-project.org/svnroot/surveillance@779 edb9625f-4e0d-4859-8d74-9fd3b1da38cb