Specify operation for target attributes

[forman]

slice_dataset.to_zarr(target_store, mode="a", ...) replaces the existing global attributes in the target target_store by the attributes of slice_dataset. The resolution of #56 is currently that we must replace the slice_dataset attributes by the attributes of the existing target dataset.

However, users should be able to select the appropriate operation, e.g., a new config setting target_attrs_op with
values "first" (default), "last", "update".