Add attributes or elements for missing seals

[larkvi]

Per @GVogeler:
From the authentication point of view a missing seal is a seal with an attribute "missing". From the textual transimission point of view, it is a physical feature (ovservable: wholes, infered: missing seal), so we could treat it with the same attribute. From the editorial point of view, people want to mark the position of the seal in relation to the text - and even with a reference to a missing seal. (SPD for Sigillum pendens deperditum). Having something to encode this would be beneficial. There was even once a dicussion on TEI-L, if I remember it correctly, where Thomas Staecker and me discussed the "L.S." abbreviation in charter copies (abbreviation for "locus sigillum").

[larkvi]

Two related but likely separate things need to be added:

1. An attribute to an marking it as missing, which would imply that the the entry was a placeholder, reconstruction, or description of a known but now lost item. Missing authen items can also be described, qua authen items, in authDesc.

2. An element or instructions in the documentation on how to re-use an existing element to mark the location where a seal would be. I would be inclined just to write about holes and the like in one of the material or support levels, but without an element describing the locus, it cannot be linked to a zone.
   Related to 2., Digital Mitford describes elements like postmarks in supportDesc https://listserv.brown.edu/cgi-bin/wa?A2=ind1505&L=TEI-L&P=R2980
   Related case: a copy has "L.S." (locus sigillorum) or the like, and the seal exists in the parent document and can be linked to using the relevant xml:id