Robustness of Daisy3 SMIL generation

We are in trouble with this kind of structure:

``` xml
begin sentence 42 after
```

DAISY3 specs expect the pagenum to have a SMIL counterpart. But if `sent3` is synthesized, the current script will create a `` node in the SMIL that links back to `sent3`. Yet `` nodes can't have any children. In particular, they can't have the pagenum as child.

So far it worked because the NLP scripts transform the structure into:

``` xml

begin sentence
42
after

```

So the Pipeline can have a different `` for `b1`, `pn1` and `a1`, though depending on NLP is not a robust approach, especially if NLP scripts are mocked for testing purposes.

When this kind of structure is detected, the Daisy3 SMIL generation algorithm should ignore the audio attached to `sent3` and deletes the corresponding mp3 from the audio-map (otherwise it will be copied to the output dir). Alternatively, perhaps it is okay with the specs to place the pagenum after the sent in the SMIL file.