Update LTI, ICR policy to most recent BoE standards

[adrian-carro]

This should include:

1. Central Bank LTI limits should be updated to the current 4.5, 15% over, thus differing from the private bank policy.
2. Computation of the fraction of mortgages over the LTI limit should be made on a 4 quarters rolling basis, as the current policy states, not every month, as currently coded.
3. Stressed interest rates for ICR calculations should be updated according to current values (see here)

[adrian-carro]

One would also need to explain this properly in the paper. For example, explain that limits are only enforced at the end of each quarter.

[adrian-carro]

Mostly solved by b934f5a814b9e9de9cdac4e47a1d3728570d269c. New updates might be introduced later on as part of a general update of the Central Bank parameter values.