added argument "weights" to tobit2Bfit() in order to allow for weighted ML estimations of tobit type-2 models with a binary outcome model

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1497 d36ae590-6c38-44c5-9ba5-7217e93607d8
added tests for weighted ML estimations of tobit type-2 models with a binary outcome model

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1498 d36ae590-6c38-44c5-9ba5-7217e93607d8
tobit2Bfit() is now exported and documented

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1499 d36ae590-6c38-44c5-9ba5-7217e93607d8