Don't use an additional data volume for half tomography

Currently, a new data volume is created for half tomography to build sinograms. This means that if a data chunk of shape (n_angles, delta_z, n_x) is read, then an additional data volume (n_angles//2, delta_z, n_x2) is created.

The extended sinogram could be built just before performing reconstruction - if building such a sinogram is needed.

Overall, lifting the current constraint would enable to process bigger chunks of data in half tomography.