EAD import: don't map <head> to note_multipart.label

On export, this leads to duplicated <head> tags, which
result in invalid EAD exports
Allow nil in ordered list enum; EAD export: nil attribute handling
EAD export: better handling for tags at start of note etc. content
EAD export: handle date['begin'].nil?
EAD import: map indexentry/ref[@target] to note_index_item.reference
EAD export: no <controlaccess> when no linked subjects or agents