fixed bug that made residuals( object, part = "outcome" ) not work if "object" was a sample selection model with a constructed variable (e.g. "log(wage)") estimated by ML (it worked for models estimated by the 2-step method)

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1540 d36ae590-6c38-44c5-9ba5-7217e93607d8
added tests for ML estimations of models based on the data set Mroz87 (basically copied the tests for 2-step estimations)

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1541 d36ae590-6c38-44c5-9ba5-7217e93607d8