Initializes a scratch pad equal in size to the wavefunction.

def _init_scratch(self):
        """Initializes a scratch pad equal in size to the wavefunction."""
        scratch = np.zeros((self._num_shards, self._shard_size),
                           dtype=np.complex64)
        scratch_handle = mem_manager.SharedMemManager.create_array(
            scratch.view(dtype=np.float32))
        self._shared_mem_dict['scratch_handle'] = scratch_handle