code: def persist(filename):
      
      dem_projected = obtain_to(filename)
      with nc.loader(filename) as root:
          data = nc.getvar(root, 'data')
          dem = nc.getvar(root, 'dem', 'f4', source=data)
          stack = [dim for dim in dem.shape if dim not in dem_projected.shape]
          stack = stack[0] if stack else 1
          dem[:] = np.vstack(map(lambda x: [dem_projected], range(stack)))
docs: Append the digital elevation map projected (using lat lon) as variables of
      the netcdf file.
  
      Keyword arguments:
      filename -- the name of a netcdf file.