Adding Profitability Index computation (PI)

[patakib]

Hello!

I added PI value calculator as a function which returns the profitability of a project.

Reference: https://en.wikipedia.org/wiki/Profitability_index

Interpretation: PI > 1: project is acceptable PI < 1: project is unacceptable PI = 1: neutral.

Thanks for the review!

[OriKatz1]

There is a mistake in the code, should be: return ((future_cashflow / (1+rate)**np.arange(1, len(future_cashflow)+1)).sum(axis=0)) / -initial_inv