Set default GSNAP vmem for GSNAP wrapper

public/biopet-framework/src/main/scala/nl/lumc/sasc/biopet/extensions/Gsnap.scala

@@ -35,7 +35,7 @@ class Gsnap(val root: Configurable) extends BiopetCommandLineFunction {
   override val defaultThreads = 8
 
   /** default vmem for cluster jobs */
-  override val defaultVmem = "6G"
+  override val defaultVmem = "32G"
 
   /** input file */
   @Input(doc = "Input FASTQ file(s)", required = true) //var input: List[File] = _