Make geotiff's IFDEntry and Tag both package-public

[WeatherGod]

The GeoTiff class has public methods that take and return IFDEntry and Tag instances. And they are both concrete implementations of the underlying spec. I don't see any reason to make them package-private. My use-case for having them public is writing integration tests for a new feature in thredds that verifies that the produced geotiff file has the expected entries and tags.