Properly translate PAGE metadata LastChange, Created, Creator

[kba]

          IMHO the correct representation would have been:

• for PAGE's Metadata/Created: a separate Description/Processing element with processingCategory=contentGeneration and the respective processingDateTime (independent of the step_alto entries for each Metadata/MetadataItem)
• for PAGE's Metadata/LastChange: a separate Description/Processing element with processingCategory=contentModification and the respective processingDateTime (independent of the step_alto entries for each Metadata/MetadataItem)

For ALTO v2 with its preProcessingStep|ocrProcessingStep|postProcessingStep distinction, one would probably have to map to:

• for PAGE's Metadata/Created: a separate Description/OCRProcessing element with ocrProcessingType=preProcessingStep and the respective processingDateTime (independent of the step_alto entries for each Metadata/MetadataItem)
• for PAGE's Metadata/LastChange: a separate Description/OCRProcessing element with ocrProcessingType=postProcessingStep and the respective processingDateTime (independent of the step_alto entries for each Metadata/MetadataItem)

But obviously, this is not ideal. However, since PAGE's Created/LastChange does not have a clear semantics, I would argue this is the best pragmatic fit.

BTW, we are also still missing Metadata/Creator! IMO this should go into the contentGeneration (or preProcessingStep) entry.

Originally posted by @bertsky in #37 (comment)