New requirement to use `surface/@n` to generate TOC for facsimiles

[martindholmes]

As outlined in projectLEMDO/lemdoIssues#355, there will now be a new @n attribute value will be added to the <surface> elements in facsimile files, consisting of:

image number with leading zeroes + pipe + signature

e.g. n="001|A1R"

These should be parsed to provide the TOC for the facsimile -- meaning that these should be parsed to generate the JSON that Monocle uses to create the facsimile TOC. The suggestion is to turn this into:

1-A1R

etc. in the TOC, by stripping leading zeroes from the page numbers.