better trendLevel.R colour scaling and allow missing to be shown in different colour

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/openair@870 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
update DESCRIPTION

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/openair@871 edb9625f-4e0d-4859-8d74-9fd3b1da38cb