ENH: Return close results in rate calculation

[Kai-Striega]

This PR changes rate to return results for all solutions found that are within tol rather than returning nan if any of the results are found. For example on master running:

import numpy_financial as npf
pv = [-593.06, -4725.38, -662.05, -428.78, -13.65]
fv = [214.07, 4509.97, 224.11, 686.29, -329.67]
print(npf.rate(2, 0, pv, fv))

returns
[nan nan nan nan nan]

the same code now returns

[-0.39920185 -0.02305873 -0.41818459 0.26513414 nan]

This PR closes gh-48