Better handling of file structures with CheckpointWrapper
Some databases have file structures very flat (with more than 30k files in a directory). This is not good for the Idiap file structure and can let our I/O super slow.
We should implement a hash function in
CheckpointWrapper.make_path that generates directory names given
sample.key to limit the number of files in a directory to 1000 files.
ping @lcolbois (this touches experiments with IJB-C)