Adds a method for setting the parent of a NuCoreFile.
NuCoreFile objects now use provided ActiveFedora functionality to assert their relationship to the parent Collection.
set_parent now returns true if it succeeds
Adds test for parent setting code in the NuCoreFile.
Updates the Nu Collections model to take advantage of ActiveFedora functionality
Update nu_core_file_spec to test that parent collection is picking up on the existence of its new child.
Renames compilation method 'entry_pids' to 'entry_ids' to conform to the defaults spun up by ActiveFedora.