Added non-standard calendar support for add_day_of_year

The `iris.coord_categorisation` function `add_day_of_year` now supports all calendars in `iris.unit.CALENDARS`. This was achieved by indexing into the tuple of time values using the index of year-day instead of the namedtuple index `tm_yday`, which is only available in `time.struct_time` instances as generated when the time unit's calendar is standard. Such value-based indexing is supported by all calendars.

Added unit test for confirming all calendars now work as expected with this coord categorisation.