Factor out moving over loci to an iterator to allow for folds

I wanted to factor out moving over loci for a set of SlidingWindows.  Until now we had only performed a flatMap over loci in a window collecting results at each loci.  I wanted to do two things 1) Factor out the code to move loci by loci over a set of sliding windows and 2) Add some other operations as we move loci by loci, i.e. a fold so we can aggregate data about the reads in order.  I will add some tests for the fold, but wanted to get some feedback earlier on.

cc @timodonnell Might be worth checking out for you if you are looking to get back into guac things.
s/7/6/?

I think you should use 1-based loci throughout this comment though, tbh; 0-based obvs better in general but it's confusing since you are setting the reads at 1, 4, and 7 below...