Implement the functions _insert_entity() and insert_reference_tile()

Implement the functions _insert_entity() and insert_reference_tile(). The first is a general function for the SQL insertion of a new entity into the entities table. Its inputs the entities' Quadkey, geometry, country ISO code, category and source IDs and the attributes dictionary. The second is a tile specific function for reference tiles, which additionally provides a tiles reference_built_area and sets the category to 0 for the tile.