added invgaussina for IWLS

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/bayesr@860 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
trying out backfitting4

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/bayesr@861 edb9625f-4e0d-4859-8d74-9fd3b1da38cb