Webdataset

- GOPEN_VERBOSE works for cache
- maybe_cached_tarfile_to_samples for making optional caches easier
- cache_dir now determines whether caching happens
- sample caching with Cached / LMDBCached

**Full Changelog**: https://github.com/webdataset/webdataset/compare/0.2.20...0.2.22

- added maybe_cached_tarfile_to_samples for simple pipeline building with caches

- changed default behavior for caching in WebDataset (cache_dir, not cache_size determines cache use)

You can use GOPEN_REWRITE to rewrite paths with regular expressions,
USE_AIS_FOR to redirect gs/s3 URLs to an AIStore instance,
and Cached/LMDBCached to store local copies of samples after
decoding.