Avoid NullPointerException when checking for R2 in Gentrap

public/gentrap/src/main/scala/nl/lumc/sasc/biopet/pipelines/gentrap/Gentrap.scala

@@ -790,7 +790,7 @@ class Gentrap(val root: Configurable) extends QScript with MultiSampleQScript wi
       )
 
       /** Convenience method to check whether the library is paired or not */
-      def paired: Boolean = Option(config("R2").asFile).isDefined
+      def paired: Boolean = config.contains("R2")
 
       /** Alignment results of this library ~ can only be accessed after addJobs is run! */
       def alnFile: File = mappingJob.outputFiles("finalBamFile")