Adding sanity checks in case of data length == 0

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/aqr@64 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Adding column classes

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/aqr@65 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Adding column classes

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/aqr@66 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Adding column classes

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/aqr@67 edb9625f-4e0d-4859-8d74-9fd3b1da38cb