Returns the NPV (Net Present Value) of a cash flow series.
Parameters :  rate : scalar
values : array_like, shape(M, )


Returns :  out : float

Notes
Returns the result of: [G51]
References
[G51]  (1, 2) L. J. Gitman, “Principles of Managerial Finance, Brief,” 3rd ed., AddisonWesley, 2003, pg. 346. 
Examples
>>> np.npv(0.281,[100, 39, 59, 55, 20])
0.0066187288356340801
(Compare with the Example given for numpy.lib.financial.irr)