Added units array to standard.json map file
Made file mode of map_file explicit
Updated add_country and add_border to accept Country params. get_country/continent_by_name no longer return None if not found
Updated create_from_map_file to read units and cards from map_file
Updated tests to account for changes to method signatures in cartogram.Map
Updated test map file to include units
Added code for testing the creation of units when using create_from_map_file
Merge branch 'feat/integrate-cards-into-maps'