added twalk MCMC propose function

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/bayesr@936 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
added twalk MCMC propose function

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/bayesr@937 edb9625f-4e0d-4859-8d74-9fd3b1da38cb