removed Tobit-5 example with simulated data in the test script heckit.R, because a very similar example is included in the test script selection.R

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1542 d36ae590-6c38-44c5-9ba5-7217e93607d8
renamed the test script heckit.R as Mroz87SelectionTest.R, because the examples in this test script all use the Mroz87 data set an they include the estimation of selection with both the heckit method and the ML method

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1543 d36ae590-6c38-44c5-9ba5-7217e93607d8