Clobbering refinement in AkomaNtoso import

We use a three-level section hierarchy for parliamentary debates: electoral term - session - sitting. There is one AN file per sitting and the files share up to two outer level sections. To import a file into an existing hierarchy and still have an opportunity to replace existing sections I propose the following two changes in AkomaNtoso import.

Clobbering works on all levels of section hierarchy now, not on the top-level section only.

There are three options for clobbering of sections with the same heading:

* `skip` to skip import of a section that already exists,
* `replace` to delete existing section and replace it with the imported one,
* `merge` to append content of the imported section to the existing one.

It might be better also to join respective command-line options into something like `--clobbering` with three allowed values `skip`, `replace` and `merge`. I just added third separate option `--merge-existing` to the existing two in sake of backwards compatibility.

I'm concerned about what's going to happen here in a document with lots of sections which don't have much in the way of headings. If you imagine a document which looked like:

```
<debateBody>
    <debateSection><speech>....</debateSection>
    <debateSection><speech>....</debateSection>
    <debateSection><speech>....</debateSection>
    ...
</debateBody>
```

containing lots of sections with no headings in them.

I think with your current code you'll either finish up with:

* the first section (skip)
* the last section (replace)
* one big merged section (merge)

When a section is completely anonymous on the headings front, I think we should always create a new one.