Report N70, N80, N90 lengths.

Note abyss-fac reports n80 lengths, and we can know what the most of the genome look like in terms of scaffold/contig sizes. fatt stat is two fold faster and report both scaffold and contig stats from single scaffold input.
It is a great win if fatt stat reported the sizes at higher coverage say N70, N80, and N90. 
This implementation is ad hoc, but I don't have a reason to write in a more general way:- say prepare arrays of reporting points.