fit.model produces less errors with check.model = TRUE

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/mptinr@125 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
fit.model even more permissive (and added this to documentation)

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/mptinr@126 edb9625f-4e0d-4859-8d74-9fd3b1da38cb