code: def _get_picard_ref(config):
      
      try:
          picard = config_utils.get_program("picard", config, default="notfound")
      except config_utils.CmdNotFound:
          picard = "notfound"
      if picard == "notfound" or os.path.isdir(picard):
          picard = config_utils.get_program("picard", config, "dir")
      return picard
docs: Handle retrieval of Picard for running, handling multiple cases:
  
      - A directory of jar files corresponding to individual commands.
      - The ubuntu/debian picard-tools commandline, which provides a shell wrapper around
        a shared jar.
  
      For a directory, configure resources with:
        picard:
          dir: /path/to/jars
  
      For the debian commandline, configure with:
        picard:
          cmd: picard-tools