get_self_ligation_threshold: Get self ligation threshold with SD method from Heidari et al
Description
This function calculates a self ligation threshold according to
the method published in Heidari et al., Genome Research, 2014.
Briefly, paired reads are divided into in evenly sized bins. For
each bin, the log2 ratio of reads that are aligned to opposite strand
vs to the same strand is calculated. Twice the standard deviation of
this ratio at high distances is used a cutoff to determine which bins
are likely to contain mostly self-liagted reads.