minor bug fix to include highest values in a bin

I noticed a few of my highest pos values weren't getting included in any of the bins, and hence needed to add a new "highest" bin.
Hi Richard, thanks a lot for spotting this, much appreciated. I think actually you could drop the `if` condition, the `stop_position` will in fact never be included as the final bin edge, even if the difference between `stop_position` and `start_position` is perfectly divisible by `window_size`.