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.
I agree with @efiring here: Let's not use six in examples unless there is a strong reason on a case-by-case basis.