filter SAM file for soft and hard clipped alignments

 Most short read aligners perform local alignment of reads to the
 reference genome. Examples includes bwa mem, minimap2, and bowtie2
 (unless in --end-to-end mode). This means the ends of the read may not
 be part of the best alignment.
 .
 This can be caused by:
 .
  * adapter sequences (aren't in the reference)
  * poor quality bases (mismatches only make the alignment score worse)
  * structural variation in your sample compared to the reference
  * reads overlapping the start and end of contigs (including
    circular genomes)
 .
 Read aligners output a SAM file. Column 6 in this format stores the
 CIGAR string. which describes which parts of the read aligned and which
 didn't. The unaligned ends of the read can be "soft" or "hard" clipped,
 denoted with S and H at each end of the CIGAR string. It is possible for
 both types to be present, but that is not common. Soft and hard don't
 mean anything biologically, they just refer to whether the full read
 sequence is in the SAM file or not.