Fix index variable overwrite and ensure that the first multiple membership ID matches the corresponding level ID for multiple membership models.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/r2mlwin@259 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Ensure rows with missing are not dropped when extracting data.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/r2mlwin@260 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Update NEWS file to reflect recent changes.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/r2mlwin@261 edb9625f-4e0d-4859-8d74-9fd3b1da38cb