modify usfm for chapter-level drafting to avoid import issues; move remarks to chapters

[mshannon-sil]

This PR addresses issue #284.

Mostly looking for high-level feedback about the approach at the moment. As we were discussing, is the right place for this functionality in the get_usfm() method as essentially a post-processing step? Or should we look to implement this feature in process_tokens() (and maybe move the remark logic here as well)?

Some initial thoughts:
Pros for putting it in get_usfm():

• The code is together in a cohesive unit making it potentially easier to maintain, rather than spread across process_token().
• If it's just for the purposes of importing, then it can be thought of as a kind of "view" that Paratext needs to avoid import issues while the true model is kept unmodified in handler._tokens. This allows for the option to access the unmodified usfm if needed in the future.

Pros for putting it in process_token():

• Faster execution time since it's all part of the same iteration
• If thought of as an essential change to the usfm structure such that alternative views are unnecessary, it could make more structural sense to include it here.

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