added internal function 'matmultdiag' to efficiently calculate diag(x %*% y). It is used by some predict methods and r.squaredGLMM.
documentation polishing

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/mumin@288 edb9625f-4e0d-4859-8d74-9fd3b1da38cb