eliminate 'trustme' argument from dmvnorm(); 
use explicit default for 'mean' and 'sigma'; analogously in dmvt() - easier code, easier for users
add test for singular Sigma

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/mvtnorm@275 edb9625f-4e0d-4859-8d74-9fd3b1da38cb