main.nf: add grouping before merge

src/main.nf

@@ -22,5 +22,5 @@ Channel.fromPath( file(params.csv) )
 workflow {
   SPLIT(fastq.r1.mix(fastq.r2))
   FASTKMERS(SPLIT.out.fastq.transpose())
-  MERGEKMER(FASTKMERS.out.csv.view())
+  MERGEKMER(FASTKMERS.out.csv.groupTuple().view())
 }