Schema change on geo and chrono metadata

Allow multiple geospatial and chronology metadata items to be associated
with subjects items.
Added stable ID app
Altered geospatial data model

Now includes identifiers for features for each item
Added event app

Events link together geospatial and chronology data. Each event takes
place at a location and during a time instant or time interval. I added
this to better model different episodes of occupation at a given site,
and model creation events at one location and findspots at another
location