Define Access Need and Access Need Group as Immutable

[elf-pavlik]

We already do it for Authorizations (Access & Data) as well as Grants (Access & Data).

Having it also for Access Need, and Access Need Group will minimize the chance of any discrepancy between Authorization / Grant and Need it satisfies.

It will also allow the Authorization to use super simple caching of all the needs that were used for generating authorizations and grants.

I don't see a reason for when Access Need and Access Need group changes, they can't get a new IRI.
We can look at supersedes links for cases that want to keep the provenance trace.

TODO

• Should a copy of access needs be retained when consent is granted? #177

[elf-pavlik]

#295 notes:

@justinwb: We may need a way to reference previous needs. For example when we want to re-authorize based on extended access needs.
@TallTed: the back-traili is useful for auditing, both individual and corporate