Fixing a bug in f_mean_hole_depth.
Fixing a bug in f_mean_hole_depth.

This method was the division of --f-hole-depths by --f-n-holes, but
hole-depths considers adjacent holes, while n-holes does not. This
resulted in a lower value than what it should have been.

badeball/tetris-ea,badeball/tetris-ea,badeball/tetris-ea,badeball/tetris-ea