Departures from TA1 schemas

[davenquinn]

Several departures from current TA1 schemas have been made for expediency and better conformance with the GeoPackage format. These changes largely don't impact the information content of the file, but make some changes to layout. We need to decide whether to update the TA1 schemas for these changes, or attempt to force better conformance to the existing schema onto this file format.

All of these changes are open to review and discussion by TA1 teams.

• extraction_identifier was renamed to extraction_pointer for clarity
• model field in several models has been renamed to pointer for clarity
• MapFeatureExtractions model is replaced by many-to-one foreign key relationships directly to the map table

• 2: line_feature and polygon_feature have been converted to MultiLineString and MultiPolygon types respectively

• 4: Delete px_geom fields in order to conform to Geopackage format

• 8: Migrate extraction_pointer to a more foreign-key based model

[davenquinn]

We are also considering migrating the extraction_pointer table to a more Foreign Key based model (#8). But I will hold off on this until consensus can be reached.