Contextual dates

[stschiff]

It'd be very good to get contextual (Archaeological) date information into the final poseidon packages. We currently only get the C14 dates, but we have lots of sites and individuals where we have not C14 date (Hallstatt-Plateau), so we just have some contextual dates. They are important, of course, so would be nice to get into Poseidon as well. There is a system, but putting contextual into the Date_Type, and then filling the Date_BC_AD_Start and Date_BC_AD_Stop from Pandoras Archaeological date range, and then just computing Date_BC_AD_Median as the mid-point.