Add integration tests for married before 2005, but not paying into pension
Add adjusted net income for marriages post 2005.
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Validate income inside the flow, not the calculator

This buys us two things:-
- It's easier to test the validation in the smart 
answer.
- If you happen to have greater pension contributions
and gift aided donations than your income, you will
have an adjusted net income of < 1, so you should
get the maximum amount. 

The existing validation is still in the calculator
to maintain backwards compatibility with V1, but
it is bypassed by a key in the hash parameter to
the initialize method.