bam: add example for paired end deduplication sorting

[kortschak]

This will look like the SortByCoordinate example for Merger but have a different less and ensure that pairs passed to the fn match QNAME.

[kortschak]

I had the thought that this would work. Further thinking shows I'm wrong. The only way to get this to work is to have a pair-ordered sort/grouping (how I did this in biogo/boom). The general algorithm is here. This would require that we add a pair-based merger.