original weights are preserved when removing NA observations from weights

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1486 d36ae590-6c38-44c5-9ba5-7217e93607d8
arguments weights is set to NULL unless it is a 2-step estimation of a tobit-2 model

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1487 d36ae590-6c38-44c5-9ba5-7217e93607d8
added tests with tobit-5 models and weights, where the weights are ignored

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1488 d36ae590-6c38-44c5-9ba5-7217e93607d8
added tests for weighted ML estimations of tobit-2 models (does not work yet as weights are currently ignored in ML estimations)

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1489 d36ae590-6c38-44c5-9ba5-7217e93607d8
added argument "weights" to tobit2fit() in order to allow for weighted ML estimations of tobit type-2 models (currently, weights are ignored in estimations of type-5 models)

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1490 d36ae590-6c38-44c5-9ba5-7217e93607d8