implemented the 'tide_system' as property in the gravity class; renamed 'permanent_tide' to 'tide_system' where relevant (the gravity class keeps the permanted_tide method); implemented the translate_tide_system and update_tide_system methods; revised the permanent_tide method
implemented the 'tide_system' as property in the gravity class; renamed 'permanent_tide' to 'tide_system' where relevant (the gravity class keeps the permanted_tide method); implemented the translate_tide_system and update_tide_system methods; revised the permanent_tide method

jgte/orb,jgte/orb,jgte/orb,jgte/orb,jgte/orb