Virtually valid

[rvanheest]

A bag is virtually complete when:

• Every required element is present (bagit.txt, data/, manifest-<alg>.txt)
• Every file listed in every tag manifest is present
• Every file listed in every payload manifest is either present in data/ or in fetch.txt
• (BagIt >= 1.0) every payload file is listed in every payload manifest
• The list of files in the payload and in fetch.txt together should make up the files listed in every payload manifest

A bag is virtually valid when:

• The bag is virtually complete
• Every checksum in every payload manifest and tag manifest has been successfully verified against the contents of the corresponding file.

Implement methods bag.isVirtuallyComplete and bag.isVirtuallyValid according to the statements above. See also LibraryOfCongress/bagit-java#126 for a little convenience with the implementation of this method. See also the description in the easy-bag-store documentation.

Perhaps we could also check for exclusive usage of ARK's as fetch items.