Excellib.irr() might not output correct values

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We use the numpy version of Excel IRR function.

But for file 230, Calculations!CU273 does not output expected value. It seems final evaluation value is correct though ...

[bleucitron]

Same for file 100032845, Calculations!AF206

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100069704, Calculations!AZ285

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100071551, Calculations!O231

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100038612, Calculations!AD206

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100035575, Calculations!W231

[bleucitron]

IRR() from Excel accepts a second argument to help the calculation to converge. If nothing is passed as 2nd arg, IRR() outputs #VALUE when it can't converge.

Numpy.irr() doesn't, and seems more efficient.