Enhance usfm2osis.py to cope with explicit end marker syntax for tags within footnotes and cross-refererences

Please take account of the implicit/explicit usage for footnotes and xrefs that Michael & I discussed a couple of weeks ago.

This aspect of USFM documentation is there in the Reference, but not so easy to find.

In the page about Syntax Notes - please see this paragraph:

The individual elements which make up the footnote or cross reference content are defined as character level markers, which means that they each define a beginning and a corresponding end marker. The Paratext translation editor will interpret the presence of a new marker as an implicit closure of any preceding character level marker. For this reason a majority of translation projects have adopted the approach of authoring footnote or cross reference content without supplying the explicit end marker. Some examples of the two acceptable markup approaches for notes is provided below (A = implicit closure; B = explicit end marker):

And then examine the given examples.