added new function 'Missing' to generate complex missing data pattern

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/lava@1071 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
added new function 'Missing' to generate complex missing data pattern

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/lava@1072 edb9625f-4e0d-4859-8d74-9fd3b1da38cb