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.


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I think if you're using multiple debateSections with no preface (so no docTitle) and no headings, then for your initial import you probably won't be using clobber/skip/etc, so that's okay, and a "reimport" would have some trouble regardless of your setting :) I think clobber could still work in this scenario - it would have to delete existing old ones that "match" (all of them in this case with no headings) and not delete ones only just inserted in the currently running import. That's what I've put in #434 anyway. Skip would also be okay (it'd skip them all), merge would be harder - but I think there's nothing really sensible it could do in such a case, you'd want to add IDs.

It should cope with multiple objects returned, yes - I think it could simply delete all it finds (clobber, see above), skip (skip), and add new (merge), in the absence of IDs.

For an initial pass though, I suggest we don't worry too much about possible cases and just have something that works for what we do have, and change as necessary in future. As long as it copes in some way with `MultipleObjectsReturned` and either just created a new section or (clobber) potentially deleted all that matched in some way, I think that would be a good improvement.