Fix YARD docs
Add TODO note for future development
Add specs to ensure the relation attribute name is set
Remove unnecessary optional argument default

* The #initialize method expects the options to include a :header
  key so the argument is required.
Rename Tuple#call as Tuple#fetch
Rename Header#call as Header#fetch
Move axiom-types to the bottom of the require statements
Change Group#each to be a bit simpler to read