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.
If we need the true read count, it should be possible to compute that in an accumulator as we assign reads to partitions, at minimal cost. If that would help, I'm happy to add that.

Also, typo around "number in th reads"