Add example working out NaN bits when working with multiple chromosomes

[jeromekelleher]

As discussed in #2083

Since there is a use-case for this now though, we could probably actually implement the functionality within msprime and do it automatically like we do for flanking regions. The main reason I didn't implement it at the time is I was worried about having lots of intervals chopped out silently, but I don't think this is a problem much in practise.