enable stdev and sstdev for groupby. fixes issue #367
enable stdev and sstdev for groupby. fixes issue #367

jmarshall/bedtools2,arq5x/bedtools2,arq5x/bedtools2,arq5x/bedtools2,arq5x/bedtools2,jmarshall/bedtools2,arq5x/bedtools2,jmarshall/bedtools2,jmarshall/bedtools2,jmarshall/bedtools2