File name expansions do not work in cfdm.write

[davidhassell]

At cfdm 1.8.9.0, tilde and environment variable file name expansions no longer work in cfdm.write:

>>> import cfdm
>>> f = cfdm.example_field(0)
>>> cfdm.write(f, 'test_file.nc')  # OK
>>> cfdm.write(f, '~/test_file.nc')  # Not OK
    ...
PermissionError: [Errno 13] Permission denied: b'/home/david/~/test_file.nc'
>>> cfdm.write(f, ''$HOME/test_file.nc')  # Not OK
    ...
PermissionError: [Errno 13] Permission denied: b'/home/david/$HOME/test_file.nc'

PR fix to follow.