adding spades 21 exit status to list

SPAdes has an 21 exit status that refers to abnormal terminations ... not always, but, must of the times it relates to memory ... thus, we added to the list of exit status to retry at least once with full allowed resources

conf/base.config

@@ -36,7 +36,7 @@ process {
       time   = {  if (task.attempt == 1) { check_max( 16.h * task.attempt, 'time' ) } else { params.max_time } }
 
       // retry at least once to try it with full resources
-      errorStrategy = { task.exitStatus in [143,137,104,134,139] ? 'retry' : 'finish' }
+      errorStrategy = { task.exitStatus in [21,143,137,104,134,139] ? 'retry' : 'finish' }
       maxRetries    = 1
       maxErrors     = '-1'
     }