MERGE_OVERLAP specifies how much of the target and query sequences must be covered by HSPs when BLASTing. BACKBONE_MIN_COVERAGE specifies the minimum number of times that a taxon must occur in an alignment
regenerated alignment
now considers overlap as specified in config file
now picks exemplars based on the frequency of their occupying most distal rank, scaled by the data set size. roughly optimally packs these taxa in a minimum of alignments