compound interest, expected loss & plotting LLR/IRB

Implemented compounding interest for discount rate calculations
Accounted for expected loss and also use it to dampen the de-risking
effect of the LLR if it is smaller than the expected loss
Disallow interest rate buydowns that result in negative interest rates,
and re-set the IRB value accordingly

Also made a new plotting script for comparing the costs of LLR vs IRB,
and saved the parameters I used to make that plot as 'runA.'