API: Add donationForRep() to Formulas

[LJNeon]

This was motivated by a player asking if there was a formulas method to calculate the donation needed to gain an amount of reputation, noting that one existed to calculate the reputation gained from a donation.

With that in mind, these methods simply implement the formulas shown in the faction UI:

ns.formulas.reputation.calculateRepToFavor();
ns.formulas.reputation.repFromDonation();

And there's already an inverse method for the first one:

ns.formulas.reputation.calculateFavorToRep();

But there was no inverse method for the second one, which is why this PR is adding this:

ns.formulas.reputation.donationForRep();

[d0sboots]

Seems good.

[jjclark1982]

related to #976

[LJNeon]

Did some basic testing and the new formulas method appears to work as intended.