Refactor calc_qualifying_years_credit and expose credit_bands method.

It will be useful to have a way of checking if a
person belongs to any of the credit bands. We only
have to ask if they did work from 16-19 if they are
Ask if you worked from 16-19 if you belong to a credit band

The years value is used in the amount result, so it
is important we ask this question.
Merge pull request #1199 from alphagov/state_pension_amends

State pension amends