output Created/LastChange timestamp as processingDateTime, fix #36

[kba]

With this PR, the alto:processingDateTime element of an alto:processingStep will be set to either the pc:Created timestamp (--timestamp-src Created), the pc:LastChange timestamp (--timestamp-src LastChange) or not at all like before (--timestamp-src none).

This is not 100% correct since Created and LastChange are document-wide and not step-specific but we have no other source for them AFAICS and it is important for our (@StaatsbibliothekBerlin) workflows to have at least an approximate date for versioning purposes in the alto:processingSteps.

[bertsky]

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.