Modify the biblStruct elements in the -md.xml files

[Conal-Tuohy]

On ingest of the legacy TEI, we should transform the -md.xml files so that the biblStruct elements are organised hierarchically, with the "readingtext" as the root and with nested relatedItem elements containing the "original_collection" and other biblStruct elements. Or, if there's no "readingtext" biblStruct then the "original_collection" is implicitly the reading text, and should be the root biblStruct with any other biblStruct elements nested within it in relatedItem elements.

Then the xinclude transclusion template files can have a much simpler structure and the stylesheet which generates them doesn't need to inspect the -md.xml file to analyse the biblStruct elements therein.

John is going to come up with a better, more explicit way to classify the different variants of the text, using @corresp to point to a declared typology, and that can be a second version of the -md.xml transformation stylesheet.

[Conal-Tuohy]

Done in fd18d9fd8af0a20edfd633d8186e51e1761dca60 including adding an xincluded taxonomy of the types of text version, which the biblStruct elements reference.

This looks correct if you look at the TEI, though the Collection facet which I updated in c6c946050246a389acdd16886395536dc7373b4c seems not to yield the number of collections I expected.

Still not quite there, but knocking off for today.

[Conal-Tuohy]

Fixed the Collection facet in 1af19e2775c6886d3bbf993c32d70111ef9bba5e