Add getYearObj() to temporal classes

Consider providing a link to the `Year` class by adding `getYearObj()` or `getYearType()` methods to all ThreeTen-Extra classes where appropriate. Check for any other similar cases that could be improved.

Note that this can be done via `temporal.query(Year::from)` or `Year.from(temporal)` already, but they seem a touch unwieldy.

Originally suggested in #155.