Financial function nper() incorrect for zero-rate annuities.

[adamrogoyski]

When rate=0.0, the sign on present value is wrong, causing most degenerate cases to be incorrect.

As an example, making periodic payments of $10 for a $100 loan at progressively smaller interest rates shows the number of periods approaching 10:

numpy.nper(0.01, -10, 100, 0)
10.588644459423231

numpy.nper(0.001, -10, 100, 0)
10.055360184319873

numpy.nper(0.0001, -10, 100, 0)
10.005503577667028

numpy.nper(0.00001, -10, 100, 0)
10.000550035678859

numpy.nper(0.000001, -10, 100, 0)
10.000055001142128

If the interest rate is 0%, it should be exactly 10 payments, not -10:

numpy.nper(0, -10, 100, 0)
-10.0

[adamrogoyski]

A fix for this:
numpy/numpy#8515