MAINT: Avoid ZeroDivisionErrors in nper_inner_loop

[Kai-Striega]

Proposed new feature or change:

I've just merged #118, this PR reproduced the behaviour of nper in Cython as closely as possible. However there are several ZeroDivisionErrors that are currently handled poorly (they return NANs).

numpy-financial/numpy_financial/_cfinancial.pyx

Lines 21 to 22 in 8583586

	z = pmt_ * (1.0 + rate_ * when_) / rate_
	return log((-fv_ + z) / (pv_ + z)) / log(1.0 + rate_)

In each case, we should carefully evaluate each code path and return a financially sensible result.