Issues in DAISY 3 to DAISY 2.02

- [ ] `par` without `text`: in DAISY 3 audio-only, a `par` without a `text` is allowed. in DAISY 2.02 it is not allowed. When downgrading, Pipeline tries to link such `par`s to a `navPoint` in the NCX (see smil2-to-smil1.xsl), but if there is no corresponding `navPoint`, no `text` tag is inserted by lack of something to reference in the NCC. A possible solution could be to merge it with the following `par`.
- [ ] `ncc:totalTime` does not equal the sum of the elapsed times of all the clips in the SMIL files: this is because somehow the number of the `audio` tags in the SMIL file is reduced during the downgrade, and the metadata isn't updated accordingly.
- [x] `ncc:maxPageNormal` gets the value `0` if there are no "page-normal" spans: this is due to a [wrong fix](https://github.com/daisy/pipeline-modules/commit/6a428e8683) of a [previous issue](https://github.com/daisy/pipeline/issues/781). (--> fixed in https://github.com/daisy/pipeline-modules/commit/1fd762d52c31bbc3566070580a859a7bf90292dc).