function probit() now has an argument "weights" for specifying observation-specific weights

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1477 d36ae590-6c38-44c5-9ba5-7217e93607d8
added tests of probit() with observation-specific weights

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1478 d36ae590-6c38-44c5-9ba5-7217e93607d8
slightly improved example in the test script for probit() models

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1479 d36ae590-6c38-44c5-9ba5-7217e93607d8