Added a test to demonstrate usage of an alternate history base class
Add base class override
Facted to allow multiple abstract bases for historical models
Extended bases override support to the register utility
Address comments
Added casting to tuple to historical 'bases' init parameter
Make bases type error more cohesive
Make sure single classes to bases fail as well
Merge pull request #98 from macro1/model-inheritance

Add override for historical model base classes
Note custom base classes in CHANGES file