biology/ad2vcf: Add allelic depth info from a SAM stream to a VCF file

ad2vdf extracts allelic depth info from a SAM stream and adds it to a
corresponding single-sample VCF file. SAM input is read via stdin and the VCF
input file is taken as a command-line argument. This allows expensive BAM/CRAM
decoding to be performed in parallel by a separate samtools process.