Unambiguous season year naming

Currently, iris.coord_categorisation.add_season_year() adds a new coordinate called 'year' to the cube. I felt this was ambiguous so I made a very small change to call it 'season_year'. I haven't changed any of the other convenience functions (e.g. day_of_year and day_of_month both make coordinates called 'day') because they are less likely to be confused, but there is an argument that they should have their names changed as well.