Implement related objects for generic files if appropriate

[elrayle]

If generic files can have related objects, then validate that the behavior is correctly inherited from pcdm:object. (See xit test Related objects -> persists in models/generic_file_spec.rb)

Otherwise, the behavior of adding related objects needs to be blocked in models/concerns/generic_file_behaviors.rb and the block tested.

[jcoyne]

I thought related objects was out of scope and that further discussion would happen before implementing it.

[jcoyne]

See discussionhttps://github.com/samvera/hydra-pcdm/pull/66

[elrayle]

My understanding from issue samvera/hydra-pcdm#66 was that related files was being postponed. Related object code was already in place and functional from PR samvera/hydra-pcdm#68. This behavior is inherited from Hydra::PCDM. Tests were added with PR #96. The ability to add related objects is now provided by services. See PR #106. Tests for related objects moved from model tests to service tests with this PR.

If GenericFiles should not have related objects, then this functionality will need to be blocked.

[grosscol]

The related objects is being addressed again during the tech call on 2015-08-05.

[mjgiarlo]

Refs samvera-deprecated/sufia#1711

[jrgriffiniii]

This is still being addressed in a forthcoming release of Hyrax: samvera/hyrax#116. It may also be deprecated by the Collections Extensions work (please see samvera/hyrax#116 (comment))