Fix issue with ElementTree when lxml is not available

I noticed this trying to get the webbpsfgui working on my Windows machine, where I don't have lxml installed.  So it tries falling back to using Python's built-in `xml.etree.cElementTree`.  However there are several places that are using `lxml.etree.Element.iterchildren` which is not supported in `xml.etree` so we have to provide an approximate replacement for it.

Once I got this going I was able to get the GUI to at least launch--not sure if there other issues but at least it runs.