DensityScale incorrectly scales power output based on air density

Currently the ratio between and site air density and power curve reference air density is used to directly scale the power output when using the DensityScale class as an additional model. This is unrealistic as the WTG generator will limit the power output if the site density is higher than the reference, and if the site density is lower than the reference, then rated power will still be reached, just at a higher wind speed. Hence it is proposed that the local effective wind speed is scaled prior to looking up the power (by a factor of (rho_site/rho_ref)^(1/3)), to effectively slide the power curve to the left or right (to lower or higher wind speeds for higher or lower site density respectively), as opposed to up or down directly based on the air density ratio. This more closely approximates the behavior of real power curves, at least until cut-out wind speed is approached and power starts to ramp down from rated power to zero, and fortunately the probability of these wind speeds is usually so low that the less realistic behavior at this wind speed range has a negligible effect on AEP.