starting values can only be used in btReg.fit if type=logit but they are really not crucial so omitted here

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/prefmod@216 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
git-svn-id: svn://svn.r-forge.r-project.org/svnroot/prefmod@217 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
improvements in plotting function

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/prefmod@218 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
improvements in worth() methods

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/prefmod@219 edb9625f-4e0d-4859-8d74-9fd3b1da38cb