Next steps for `fgfa window-depth`

[sampsyo]

Here are a few ways we can continue to improve the fgfa window-depth command, which recently landed in #263:

• The weighted_depths function could probably return an iterator to avoid materializing a full Vec. Window_depth next steps #265
• It may be possible to simplify the core assign_depths algorithm by turning the inner for loop into a while loop, eliminating the i variable, and rethinking overlap_flag. (This would not be an optimization; it would just be for readability.) Window_depth next steps #265
• We can try harder to match the exact output from odgi. We currently differ a little bit, ether in the calculations themselves or the printing.