cfdm.write "compress" parameter doesn't work for field with string-type coordinates (netCDF4==1.6.0)

[davidhassell]

Using the netCDF file cfdm/test/test_file.nc (created by cfdm/test/setup_create_field.py), that has vlen string type coordinates, and netCDF4 version 1.6.0 we get:

>>> import cfdm
>>> f = cfdm.read('test_file.nc')[0]
>>> cfdm.write(f, 'delme.nc')               # OK
>>> cfdm.write(f, 'delme.nc', compress=4)   # Not OK
Traceback (most recent call last):
  File "/home/david/NCAS-CMS/cfdm/cfdm/read_write/netcdf/netcdfwrite.py", line 2670, in _write_netcdf_variable
    self._createVariable(**kwargs)
  File "/home/david/NCAS-CMS/cfdm/cfdm/read_write/netcdf/netcdfwrite.py", line 2402, in _createVariable
    g["nc"][ncvar] = g["netcdf"].createVariable(**kwargs)
  File "src/netCDF4/_netCDF4.pyx", line 2838, in netCDF4._netCDF4.Dataset.createVariable
  File "src/netCDF4/_netCDF4.pyx", line 4003, in netCDF4._netCDF4.Variable.__init__
  File "src/netCDF4/_netCDF4.pyx", line 1965, in netCDF4._netCDF4._ensure_nc_success
RuntimeError: NetCDF: Filter error: bad id or parameters or duplicate filter

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/david/NCAS-CMS/cfdm/cfdm/read_write/write.py", line 522, in write
    netcdf.write(
  File "/home/david/NCAS-CMS/cfdm/cfdm/decorators.py", line 171, in verbose_override_wrapper
    return method_with_verbose_kwarg(*args, **kwargs)
  File "/home/david/NCAS-CMS/cfdm/cfdm/read_write/netcdf/netcdfwrite.py", line 4730, in write
    self._file_io_iteration(
  File "/home/david/NCAS-CMS/cfdm/cfdm/read_write/netcdf/netcdfwrite.py", line 5002, in _file_io_iteration
    self._write_field_or_domain(f)
  File "/home/david/NCAS-CMS/cfdm/cfdm/read_write/netcdf/netcdfwrite.py", line 3586, in _write_field_or_domain
    coordinates = self._write_auxiliary_coordinate(
  File "/home/david/NCAS-CMS/cfdm/cfdm/read_write/netcdf/netcdfwrite.py", line 2143, in _write_auxiliary_coordinate
    self._write_netcdf_variable(
  File "/home/david/NCAS-CMS/cfdm/cfdm/read_write/netcdf/netcdfwrite.py", line 2694, in _write_netcdf_variable
    raise RuntimeError(message)
RuntimeError: Can't create variable in NETCDF4 file from <AuxiliaryCoordinate: greek_letters(10) > (NetCDF: Filter error: bad id or parameters or duplicate filter)
>>> 

Everything is fine for netCDF4<1.6.0.

This is related to Unidata/netcdf4-python#1175 (comment).