#417 search-for-xlinks does not handle ties properly in computing delta_cn

[William S Noble]

When search-for-xlinks enocunters a tie in XCorr score, it reports both matches. This is correct. However, the delta_cn values are different. In this file:

http://noble.gs.washington.edu/~wnoble/proj/crux-projects/2016gaussian/results/bill/2016-07-24xlink-candidate/ribo.require-xlink=T/search-for-xlinks.txt

the first two lines correspond to matches to scan 22. Both have an xcorr score of 0.039691944, but the first has a delta_cn of 0, and the second has a delta_cn of 0.00029008836.