Added function hess2cov that computes the corrected hessian given an (approximate) hessian subject to linear equality constraints; the constraint matrix is passed as argument next the hessian.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/depmix@641 edb9625f-4e0d-4859-8d74-9fd3b1da38cb