added tests for Tobit-5 models with the same explanatory variables in both of the outcome equations

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1594 d36ae590-6c38-44c5-9ba5-7217e93607d8
added tests for Tobit-5 model with the same explanatory variables in both of the outcome equations estimated by the 2-step method

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1595 d36ae590-6c38-44c5-9ba5-7217e93607d8
minor correction in a few tests for Tobit-5 model with the same explanatory variables in both of the outcome equations estimated by the ML method

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1596 d36ae590-6c38-44c5-9ba5-7217e93607d8
fixed bug in 'selection( ..., method = "model.frame" )' for Tobit-5 models (missing "drop = FALSE") that caused model.frame() to return an incorrect variable name if the model is estimated by the ML method and all explanatory variables of the second equation of the outcome model are already included in the first equation of the outcome model; the incorrect variable name caused model.matrix(), residuals(), fitted(), and predict() to fail

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1597 d36ae590-6c38-44c5-9ba5-7217e93607d8
removed try() commands that are no longer necessary

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1598 d36ae590-6c38-44c5-9ba5-7217e93607d8
NEWS: added changes that have been implemented and that will be available in the next version

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1599 d36ae590-6c38-44c5-9ba5-7217e93607d8