ATAC-seq pipeline exit if no mitochondrial reads are duplicated

[cschmidl]

ATAC-seq pipeline exit if no mitochondrial reads are duplicated due to zero division error - see logfile: File "/Users/christianschmidl/src/open_pipelines/pipelines/atacseq.py", line 359, in parse_duplicate_stats prefix + "duplicate_percentage": (float(duplicates) / (single_ends + paired_ends * 2)) * 100} ZeroDivisionError: float division by zero