Implied rate (Short ROI) is negative in some cases where fixed > variable

[MazyGio]

The calc for implied rate doesn't currently account for backpaid interest. It's based on the full amount of basePaid, when we should be working with basePaid - backpaidInterest, since the user gets it back when closing the short, so it's not really a "cost":

backpaidInterest = bondAmount * (vaultSharePrice/openVaultSharePrice) - bondAmount