binning_z: crash when reconstructing last volume chunk

Related to #167 (closed).

When reconstructing a full volume with binning_z > 1, the volume is processed by chunks (unless everything fits in GPU memory).

In some situations, the last chunk is not "big enough" for the z-binning, and the error described in #167 (closed) occurs.

A simple workaround can be to decrease z_min for the last chunk, and to increase (upper) z-margin accordingly. By doing so, a chunk "big enough" is processed, and the excess part is discarded by the phase margin.