[Feature] Share the hologram FFT between reconstructions

• Added HologramFFT, which computes the real-to-complex FFT of a hologram once and keeps it as half_spectrum. It can be passed to reconstruct_frame() and reconstruct_bf() in place of the hologram, so that reconstructing both the complex wave and the brightfield image only needs a single FFT.

• reconstruct_stack() and HoloReconstructUDF share the FFT between the wave and the brightfield image of each hologram.