added tests of nObs.probit()

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1512 d36ae590-6c38-44c5-9ba5-7217e93607d8
added a set.seed() command in the test script invMillsRatioTest() so that the data and results are always the same

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1513 d36ae590-6c38-44c5-9ba5-7217e93607d8
objects returned by binaryChoice(), probit(), and summary.probit(): renamed component "df" as "df.residual" (degrees of freedom of the residuals) in order to be consistent with other R functions

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1514 d36ae590-6c38-44c5-9ba5-7217e93607d8
added tests for applying the df.residual() method to objects returned by probit(); df.residual.default() works, because the degrees of freedom of the residuals are in component df.residual (see previous revision)

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1515 d36ae590-6c38-44c5-9ba5-7217e93607d8