Deal with edge case better

The highest bin is likely to be enriched for zelda (though it's about
hte only one), and the highest of the highest even more so, so by
reversing it, I'm avoiding shrinking the size of the bin artificially.

Note that there is no problem with the lowest different bin.
Merge branch 'ZeldaAnalysis' of github.com:petercombs/EisenLab-Code into ZeldaAnalysis