(17B/19B) GPM: initial payment

[Albert Chan]

Hi, Gil

We need bank APY because payment growth is on yearly basis, but compounding is monthly.
If payment growth is also on a monthly basis, we don't need effective rate.

lua> s, i, pv = 1.075, 0.01, 6e4
lua> USPV = fn'i,n: -expm1(log1p(i)*-n) / i' -- = 1 / cn
lua> s1 = tvm(12, nil, -1, 0, s) -- APY to monthly rate

q = (1+s1)/(1+i) --> q-1 = (s1-i)/(1+i)

pv/pmt = (1 + q + q^2 + ... + q^59) / -c1 + q^60 / -c300

lua> tvm(60, (s1-i)/(1+i), USPV(i,300), USPV(i,1), nil) -- = pv/pmt
-128.065884851857
lua> pv / _ -- = pmt
-468.50884659412856