Fix python3 issues in some examples

Unfortunately these are the easy ones ...

This mostly involves intervals/keys and xrange. For the most part I chose simplicity over minimising memory usage and replaced interkeys etc. with keys. The loops are rather short so the benefits from iterators is rather limited. If anyone thinks it is important I am happy to change this.
We don't test it but afaik we haven't actually on purpose introduced code that breaks support. In any case this is only an example so much less important than core code to remain compatible with old versions