Compute dynamic read depth limits

This builds on #189 

Here were use the `windowFoldLoci` to sum up aggregate statistics on the read (average depth, average insert size)  Later I want to change these to be more complete distributions (so we can computer std dev, percentiles).  

There is a new command line option `-approx-depth-limits` which estimates the max and min read depths using the sample statistics.
shadowing `i` here?