Fixed interleave-reads.py's optional file issue.
Fixed interleave-reads.py's optional file issue.

F1000Research/khmer,Winterflower/khmer,ged-lab/khmer,ged-lab/khmer,kdmurray91/khmer,Winterflower/khmer,souravsingh/khmer,F1000Research/khmer,ged-lab/khmer,Winterflower/khmer,jas14/khmer,jas14/khmer,kdmurray91/khmer,F1000Research/khmer,souravsingh/khmer,kdmurray91/khmer,souravsingh/khmer,jas14/khmer