Quickmerge with more sensitivity and accuracy

In this version of quickmerge, we fixed some minor bugs to make the program more robust to incongruence between the component assemblies. We made a subtle change in the algorithm to markedly increase its sensitivity. With the new quickmerge, 52X Drosophila melanogaster reads generate an assembly with 16Mb N50, not very far from what Berlin et al. (2015, Nat. Biotech) got with 120X reads (previous version would get 9.1Mb N50). This version also introduces a new parameter to reduce errors introduced by repeats.