Remove NoNullGuessingAssertionModel.
Extract GuessingObjectChooser from AgentImpl.
Create interface and basic implementation for GuessingObjectChooser.
Pass GuessingObjectChooser into AgentImpl constructor.
Move regeneration implementation into AgentImpl.
Write test for BasicGuessingObjectChooser.
Create NoNullGuessingObjectChooser.
Make GuessingAssertionModel return Assertion rather than
GuessingAssertion.