code: @Deprecated
    public void concat(Path trg, Path [] psrcs) throws IOException {
      concat(trg, psrcs, true);
    }
docs: THIS IS DFS only operations, it is not part of FileSystem
  move blocks from srcs to trg
  and delete srcs afterwards
  All blocks should be of the same size
  @param trg existing file to append to
  @param psrcs list of files (same block size, same replication)
  @throws IOException