Override writeObject/readObject in TileIdWritable
Using the defaults is wrong since it results in an uninitialized member "0" on the read side
Get the filesystem using the path rather than FileSystem.get(conf)
GeneratePartitionedMapFiles fixes
1. Write out the splits file after all the parts have been written
2. Write out the parts using the Hadoop "part-XXXXX" lex ordering scheme.
This scheme is critical for the partitioner to work.