Allow adding GenericFile to GenericWork in memory (without saving either object)

[flyingzumwalt]

This is the best example I could find with a quick search in sufia-core. There are definitely more in curation_concerns.

sufia-core catalog_controller.rb
Had to rewrite this, which never actually saves the GenericWork and GenericFile objects
https://github.com/projecthydra-labs/sufia-core/blob/migrate_to_curation_concerns/spec/controllers/catalog_controller_spec.rb#L114-L135
and replace it with this:
https://github.com/projecthydra-labs/sufia-core/blob/migrate_to_curation_concerns/spec/controllers/catalog_controller_spec.rb#L114-L135

[flyingzumwalt]

@elrayle @jcoyne

[grosscol]

Getting the associations without a call to save is the primary hurdle? Is the goal of this to speed up tests?

[tpendragon]

I think it's more than just tests - you should be able to build things up in memory before persisting, and right now you can't. Seems related to samvera/active_fedora#858