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.
you have two `i`s in scope at once, if i'm reading this right; the second one is said to "shadow" the first, or vice versa, I forget, but it's generally good to avoid doing, esp. since it's typically cheap to just rename one of the variables.

fwiw i believe `(0 to 25).map(_ => "TCGA")` would do what you like and maybe makes it more clear that the mapped variable is not being used.