curation_concerns_type=() should raise if called twice

[atz]

CC logic is highly dependent on curation_concerns_type=. Unfortunately that method relies on CanCan side-effects via load_and_authorize_resource (namely the addition of before_actions).

Code in question here:
https://github.com/projecthydra/curation_concerns/blob/aa46cbd2da3123ffbf11612395f0bbc8dbc613ac/app/controllers/concerns/curation_concerns/curation_concern_controller.rb#L23

The ramification of this pattern is that inheritance in downstream controllers (where a child might want to override the type) is impossible to do cleanly. The filters are aggregated because of LAAR, meaning the security is broken.

At a minimum, CC should check in curation_concerns_type= to see if _curation_concerns_type is already set and raise if so, since the results of compounding side-effects cannot be what the user desires. (I think it would even result in curation_concern being set twice during a request.)

More broadly, I would love to see a reworking by which curation_concerns_type= is replaced so that inheritance can still be practiced with downstream controllers. (The timing of the LAAR is still tricky though.)