Add precise C14 data

[jfy133]

@DianaSpurite is already starting on this offline, but also with the additional purpose of having a separate paper that we assess the quality of radiocarbon date reporting.

We can use the information Diana intitially collects, but then we should pare this down to just the essential columns for the actual Dir.

I will start writing a schema for the whole quality-assessment data, and then when we merge this in properly we should drop the unnecessary columns.