Make ZipperBams hanlde program name collisions

[bwlang]

I had to add --no-PG to my ubam generation code to avoid name collisions.
to reproduce create a ubam with samtools cat and samtool collate.
then use samtools collate to product the aligned bam.

[bwlang]

here is the specific error:
java.lang.IllegalArgumentException: Program record with group id samtools already exists in SAMFileHeader!