Correct previous check-in to use levID generated from the formula object.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/r2mlwin@280 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Check the object parameter is an mlwinfitIGLS or mlwinfitMCMC class.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/r2mlwin@281 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Correct residual calculations related to standardised residuals.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/r2mlwin@282 edb9625f-4e0d-4859-8d74-9fd3b1da38cb