merged documentation of fitted.probit() and nObs.probit() into one documentation file probit-methods.Rd; added documentation to fitted.probit() and print.probit() to this documentation file; mentioned default methods that can be applied to "probit" models; added references to other methods for "probit" models that are described in separate documentation files

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1519 d36ae590-6c38-44c5-9ba5-7217e93607d8
added tests of further methods for "probit" models

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1520 d36ae590-6c38-44c5-9ba5-7217e93607d8