This one should work in the unmerged graph, directly after the graph building. 

My assumption here is that if the offset come from the `unflippedset`, then the `offset` should always `<=0`. Because the direction is the same, and the only shift sequence allowed should be `-1 --> -2 --> -3 --> ...`. And the correspond `flippedset` should always be `>=` `kmerlength-1`. 

I was using the neighborlist before, then I found it really tedious to play with `FF, FR ...`, and also I need to compare the dest with every info inside the ReadHeadSet to get the proper ones. Then I decide to change to directly loop through the `ReadHeadSet` which is more deterministic.