getBorrowRate

params of rateParams -> callback(response of yAssetParams)

calculates $BorrowRate$ against params

$$U(x) = UtilisationRate(x)$$

$$BorrowRate = \begin{cases} baseRate + U(params) * multiplier & \text{if } U(params) \leqslant kink\\ baseRate + kink * multiplier + (U(params) - kink) * jumpMultiplier & \text{if } U(params) > kink\\ \end{cases}$$