This function checks the position of the SNP within the trimmed sequence tag
and identifies those for which the SNP position is outside the trimmed
sequence tag. This can happen, rarely, when the sequence containing the SNP
resembles the adaptor.
The SNP genotype can still be used in most analyses, but functions like
gl2fasta() will present challenges if the SNP has been trimmed from the
sequence tag.
Not fatal, but should apply this filter before gl.filter.secondaries, for
obvious reasons.