Only generate usfm related to requested chapters if user specifies chapter-level drafting

[mshannon-sil]

For the incremental draft generation feature, SILNLP needs the ability to produce usfm files that only contain a \id marker and then the relevant usfm for each chapter drafted, as described in this issue sillsdev/silnlp#836. This is to prevent overwriting problems when importing the generated draft into Paratext. In machine.py, there needs to be changes to the update_usfm method to enable this behavior. This feature might not be needed for Scripture Forge, since SF has its own logic to select only the chapters the user requested.

Additionally, AI drafting remarks need to be placed after each chapter marker now, rather than simply once at the beginning of the book.