Abstract class for object with "get_factor_by_name" method

[mscarey]

This class could be inherited by Opinion, and then it would provide a type annotation for class methods that can take an Opinion as a parameter in modules that can't import Opinion directly.

[mscarey]

All AuthoritySpoke modules can now import Opinion directly, because it's imported from Justopinion. Beyond that, I don't want to propagate the get_factor_by_name method any further than it's already gone.