Add forwarded losses and fix NPV per #180

[yalinli2]

See details in #180

[yoelcortes]

@yalinli2,

Cool! Thanks for making these changes. I made the following updates:

1. I separated the taxable_earnings calculation from forwarded loses to optimize methods like solve_price and solve_IRR which run NPV calculations multiple times.
2. I changed the name pay_interest_during_construction with accumulate_interest_during_construction for more clarity. If accumulate_interest_during_construction is True, the interest is payed later as part of the loan principal. If False, the interest during construction is payed immediately (in this case by equity).

Using your last test in #180, I get the same results.

Thanks so much!