Renamed TIFFPyramidLevelList to TIFFImageList and renamed TIFFPyramidLevel
to TIFFImage. Also renamed method TIFFImage.getLevelNumber() to
TIFFImage.getImageNumber().
Updated BigTIFFLon8TilesTest to work with refactored code.
Removed references to a label from TIFFImageList.
Added reporting of number of tiles written as well as the time taken.
Refactored code so that TIFFImage is now abstract and added two new classes
which are subclasses of TIFFImage: MultiTileTIFFImage and
SingleTileTIFFImage.  SingleTileTIFFImage currently throws an
UnsupportedOperationException as this is not yet supported.
Rename Tile to Image, JPEGTile to JPEGImage and RGBTile to RGBImage.
Refactored code to separate out the retrieval of image data (tiled vs. non-
tiled) from the creation of a particular image type (JPEG vs. RGB). Non-
tiled images still not supported yet.