Core: implement scaling of partial intensities

The new rescaling workflow (!99 (merged)) assigns each peak to the image nearest to its peak centre, and gives it the scaling factor for that image. Would it be more accurate to scale the partial intensities and sum them to get the corrected intensity? How would this affect the calculation of Peak3D::correctedSumIntensity?

The partial intensities are already stored in Peak3D::_rocking_curve, albeit with incorrect sigmas, but only for PixelSumIntegrator.