mods:recordIdentifier not wrapped in mods:recordInfo

[mikegerber]

Example file: 24350382_1892-11-13_0_0.xml

Some newspaper METS doesn't have the mods:recordIdentifier wrapped in mods:recordInfo. We currently handle this in a hacky way (trying the correct recordInfo_recordIdentifier first as index, and use the "bare" recordIdentifier on exception).

→ Fix auomatically instead and throw a warning.

[mikegerber]

Relevant changes are now merged to master. For now, we just use recordIdentifier-zdb instead of the correct recordInfo_recordIdentifier-zdb.