Add method for shifting of convolution kernel as function of fDC

 - for optimal resampling of SAR StripMap data non-centrality of spectra in Azimuth direction
 has to be accounted
 - instead of shifting data back and forth, spectra of the interpolation kernel is shifted
 - this shifting is achieved by multiplication of the kernel, in space domain, with a phase trend of -2pi*(time)*fDc/PRF