[Misc] Threaded frame registration
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 * :func:`~libertem_holo.base.convenience.reconstruct_stack` registers and
   shifts the frames of a stack in a thread pool (:code:`align_threads`,
   default: the number of CPUs for numpy, 1 for cupy; the threads use the
   caller's current GPU), with BLAS limited to one thread meanwhile.
   Aligning 30 frames of 480x464 pixels takes about a tenth of the time it
   did; the results are unchanged.