use pb/@facs for image but pb/@corresp for presentation URI

[bertsky]

Perhaps lb/@facs would have been more consistent, because DTABf does not use either, but uses pb/@corresp as file URI and pb/@facs as identifier, as do other dialects.

Perhaps, we could also use a facsimile section with one surface per file that contains all zone segments including coordinates and segment type?

Originally posted by @bertsky in #66 (comment)

[bertsky]

Best of both worlds:

• use pb/@facs for images with a non-DTABf facsimile section with a graphic child for every pb that contains the actual @url and defines an xml:id usable for @facs
• use pb/@corresp for the digital presentation URI, i.e. mods:identifier[@type="purl"] (which could in theory be combined with path suffixes to highlight text search results)