USFM markers for cross-references

[DavidHaslam]

It would be useful to mark all the cross-references in the text using the proper USFM tags \x_...\x*

Then they would be amenable to being converted to links in Bible software.

Refer to the USFM User Reference for details.

[DavidHaslam]

This prompts the question,

Do we need to retain the parentheses that wrap the cross-references, once they have been properly marked?

IMHO, these parentheses should be removed.

[DavidHaslam]

I have just made and tested a bespoke TextPipe filter to mark the 2534 cross-references.

It was designed to avoid changes to parenthesised items other than an xref. There are 214 such items.

It will also fix a further type of malformed xref. See #2

• With 2 periods at the end of a book abbreviation