By introducing the use of h5netcdf with the h5py backend as a backend option to cf.write, we open up being able to control the HDF5 internal metadata structure via the full range of h5py.File keyword arguments (https://docs.h5py.org/en/stable/high/file.html#h5py.File), which can be set with the new h5py_option parameter to cf.write.

For now, the h5py_option parameter will be the only way to control these option, but in the future we could make some common options available directly through the cf API.

Also in the future, we may well want to set some of these HD5 options automatically (be default) based on the nature of the fields being written out.

This will be almost entirely implemented and inherited from cfdm.

PR to follow.