Don't duplicate SxS for headers with multiple labels; add a labels field

When encountering a SxS for "Paragraphs 33(b) and (c)", the code used to just copy the SxS. This leads to confusing bugs where the header contained subheaders, resulting in a single paragraph's SxS appear multiple times.

This solution is much cleaner. It replaces the "label" field with "labels", allowing a single SxS node to correspond to more than one label.