whitespace
flatten_hard_refs should not silently allow nonsense UR::Object refs

We encountered an issue with flatten_hard_refs silently allowing garbage rules
to be created by passing over UR::Objects with missing data_types.  This
prompted a cleanup of the method.
rename variables to clarify intent of code
Merge pull request #34 from nnutter/flatten_hard_refs

flatten_hard_refs should not silently allow nonsense UR::Object refs