Add isTiled() method for run-time test if a raster is tiled.

At the moment, we rely on a run-time pattern match on a raster's
data to determine if a raster's underlying data is tiled or not.

The isTiled() method allows us to do that without reflection.
Use isTiled instead of pattern matching on RasterData type.
Use isTiled() isntead of pattern matching on type.
Merge pull request #392 from joshmarcus/topic/is_tiled

Add isTiled() method for run-time test if a raster is tiled.