Clarify that there MUST be an inventory.json file in the Object Root

[ahankinson]

We have decided to make the presence of an inventory.json file optional in the version directories (3.7 "Every version directory SHOULD include an inventory file that is an Inventory of all content for versions up to and including that particular version.") However, we don't also specify a MUST for the inventory.json file in the root of the object.

This can lead to the absurd situation where an OCFL Object without an inventory.json file is technically valid!

Propose modifying §3.7 to read:

"Every OCFL Object MUST have an inventory file within the OCFL Object Root, corresponding to the state of the OCFL Object at the current version. Additionally, every version directory SHOULD include an inventory file that is an Inventory of all content for versions up to and including that particular version. Where an OCFL Object contains inventory.json in version directories, the inventory.json in the OCFL Object Root MUST be the same as the file in the most recent version."