removed "weights" from creating model frames

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1480 d36ae590-6c38-44c5-9ba5-7217e93607d8
removed "weights" from creating model frames (now also in selection() and tobit2())

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1481 d36ae590-6c38-44c5-9ba5-7217e93607d8
added a set.seed() to make later examples independent of changes in random number generation in earlier examples

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1482 d36ae590-6c38-44c5-9ba5-7217e93607d8
added argument "weights" to selection() and heckit2fit() (currently, this only works for 2-step estimations of tobit-2 models)

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1483 d36ae590-6c38-44c5-9ba5-7217e93607d8
added tests of 2-step estimations of tobit-2 models with weights

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1484 d36ae590-6c38-44c5-9ba5-7217e93607d8