cdm/src/main/java/ucar/nc2/iosp/netcdf3/N3iosp.java
@Override
  public void openForWriting(ucar.unidata.io.RandomAccessFile raf, ucar.nc2.NetcdfFile ncfile,
                   ucar.nc2.util.CancelTask cancelTask) throws IOException {
    open(raf, ncfile, cancelTask);
  }
@Override
  public void openForWriting(ucar.unidata.io.RandomAccessFile raf, ucar.nc2.NetcdfFile ncfile,
                   ucar.nc2.util.CancelTask cancelTask) throws IOException {
    open(raf, ncfile, cancelTask);
  }
read existing file