Changes to XML processing for amendments

Penultimate batch of changes for interp processing in amendments

* Normalize XML that uses regtext-like tags in the interpretation sections
* Catch interp "headers" like `<P>44(e)(2) Something</P>` (before we needed HD tags or emphasis)
* Use regex for interp paragraph markers (bit faster)
* Allow "i * * *" as a paragraph marker (note the absence of a period)
* Add a few exceptions for collapsed markers (such as "i.e.")
* Append interpretation paragraphs with no marker to the preceding paragraph. In appendices, we assign a label, but as interpretations are *supposed* to be more structured, we'll assume it's a typographical error
* Skip over PRTPAGE tags when finding the "next" marker in regtext
* Don't demand the *next* marker follow a paragraph marker to disambiguate it (e.g. allow "iv" to distinguish "i"). This helps when dealing with missing parts of the paragraph, such as we find with amendment parsing.