#212 bogart load balancing

[Brian Walenz]

Bogart needs to scan the list of unitigs to find those that will have work to do during repeat detection, and then partition only those unitigs among the compute threads. It currently partitions all unitigs among the threads. But since most short unitigs are at the end of the unitig list, these threads really have nothing interesting to do. Most of the work is done by a few threads.