Phase Retrieval: support non-square pixel size

For now PaganinPhaseRetrieval does

        fy = np.fft.fftfreq(nyp, d=self.pixel_size_micron)
        fx = fftfreq(nxp, d=self.pixel_size_micron)

We should be available to provide a tuple for pixel size.

The same goes for CTFPhaseRetrieval:

        fsample_vh = np.array(
            [
                self.geo_pars.length_scale / self.geo_pars.pix_size_rec,
                self.geo_pars.length_scale / self.geo_pars.pix_size_rec,
            ]
        )