Prune data in identity and H_tilde_rec

Inside block_diagonalization.general we define identity and H_tilde_rec as auxiliary series to compute U. These don't need to store their data and reduce the memory footprint, but they do now. We could, for example, delete the corresponding data entries immediately after usage if we determine that it's safe. This might require extending the BlockSeries API because right now we consider data a private attribute.