added tests with variables directly in the work space (rather than in a data.frame)

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1561 d36ae590-6c38-44c5-9ba5-7217e93607d8
extended tests with dummy variables as explanatory variables

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1562 d36ae590-6c38-44c5-9ba5-7217e93607d8
added tests for fitted.selection( , part = "selection" ) and residuals.selection( , part = "selection" ) for models, where the dependent variable of the selection model is a factor: residuals.selection( , part = "selection" ) does not work for these models estimated by ML (it works for the 2-step estimation method)

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1563 d36ae590-6c38-44c5-9ba5-7217e93607d8