Refactor state pension topup next node flow
Update state pension topup calculator

- Added additional calculations for up to 3 rates and ages
- Number of rates shown dependent upon age at top up start date
- Separation of calculations into new class
- Additional unit and flow test coverage
- Updated yml file to accomodate new calculations
add 'either' to outcome bullet list to avoid confusion
Merge pull request #954 from alphagov/state-pension-topup-fix

State pension topup fix