Postfit fails if there is a dot in the fit name, but the only thing that fails is the check. The rest of the code seems to do just fine. This is due to the fact that with_suffix substitutes the suffix when we actually want to append_suffix.
If we actually want to avoid any fits with dots in the name, let me know. I'll change this PR so that it actually checks for dots from the very beginning (and then it fails and it tells you why).
