>>> schema = ts.Schema()
>>> print(schema.codec)
None
>>> schema.update(codec=ts.CodecSpec({
...     'driver': 'zarr',
...     'compressor': None
... }))
>>> schema.update(codec=ts.CodecSpec({'driver': 'zarr', 'filters': None}))
>>> schema.codec
CodecSpec({'compressor': None, 'driver': 'zarr', 'filters': None})