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@1495 d36ae590-6c38-44c5-9ba5-7217e93607d8
added argument "maxMethod" to heckit2fit() and heckit5fit() that specifies the maximisation method that is used for estimating the probit model (1st stage)

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1496 d36ae590-6c38-44c5-9ba5-7217e93607d8