The purpose of this is to be able to call SNPs that fall at the beginning of a capture region (or exon).
Before, the read threading code would only start threading from the first kmer that matched the reference. But
that means that, in the case of a SNP at the beginning of an exome, it wouldn't start threading the read until
after the SNP position - so we'd lose the SNP.
For now, this is still very experimental. It works well for RNAseq data, but does introduce FPs in normal exomes.
I know why this is and how to fix it, but it requires a much larger fix to the HC: the HC needs to pass all reads
and bases to the annotation engine (like UG does) instead of just the high quality ones. So for now, the head
merging is disabled by default.
As per reviewer comments, I moved the head and tail merging code out into their own class.
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