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.
Unclear from the code how many elements are in Line2D.markers so in this case I think that it makes sense to use an iterator to avoid 2 copies of the list of makers.