corrected the calculation of one element of the hessian matrix of the log likelihood function for tobit-2 models with weights

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1491 d36ae590-6c38-44c5-9ba5-7217e93607d8
made the calculation of the gradient of the log likelihood function of tobit-2 models more robust

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1492 d36ae590-6c38-44c5-9ba5-7217e93607d8
made the calculation of the gradient and Hessian of the log likelihood function of tobit-2 models numerically more robust

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1493 d36ae590-6c38-44c5-9ba5-7217e93607d8
made the calculation of the Hessian of the log likelihood function of tobit-2 models numerically more robust

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1494 d36ae590-6c38-44c5-9ba5-7217e93607d8