Create a cache for file info

[pieterdavid]

(with verbose it is otherwise done three times before processing the samples)

This is not the cleanest (it could be done with a function decorator) nor safest (it assumes the list of files doesn't change, which is reasonable but not paranoid; in principle the cache should be locked when updating, to avoid corrupting the file if written from several processes simultaneously) implementation, but should be a good speedup in any case