fix iterator for sequence handle

This bug actually lead to the discovery of an interesting
phenotype of the "for" statement. If a class has the
__getitem__() defined and doesn't have a __iter__(),
Python uses __getitem__() to iterate over the items,
stopping when a IndexError is thrown. Previously we were
throwing a normal RuntimeError which Python didn't recognize
as the end of the list.

git-svn-id: https://dng.biozentrum.unibas.ch/svn/openstructure/trunk@2467 5a81b35b-ba03-0410-adc8-b2c5c5119f08