add sam as hisat2 output

tasks/hisat2.wdl

@@ -22,6 +22,7 @@ task hisat2 {
   }
 
   output {
+    File sam=sample_id + ".sam"
     File unmapread_1p=sample_id + "_un.fq.1.gz"
     File unmapread_2p=sample_id + "_un.fq.2.gz"
   }