Update of stats.py (#305)
Update of stats.py (#305)

* Bug fix

If there are many GFF under a same directory and we run `python -m jcvi.annotation.stats genestats`  for each of them, we will get unexpected result because the origin code will generate one "transcript.sizes" file. It is better to generate "transcript.sizes" with the filename of GFF as prefix.

* bug fix: gene without transcripts

* ZeroDivisionError: float division by zero

Some gff file don't have exon feature,
tanghaibao/jcvi