Trigger a collection export and download it

It's possible through the Transkribus interface to ask for an export of a collection, and later get a ZIP archive.

It could be really useful to use that as source for the import into Arkindex.

I don't know much:

• it's pagexml
• it does not seem possible to trigger it through the web interface
• i don't think there are any images in there (URLs I hope)

The method should:

1. trigger the export
2. check if the export is ready, and wait a bit if not
3. download the export when available
4. support a timeout, to avoid looping indefinitely