Release 2.0.0 beta 5

Changes from 2.0.0-beta.4 to 2.0.0.beta5

• The prefilter functionality has been introduced and declared stable.
  With that, you can callback arbitrary functions previous to any filter.
  This can be useful for performing (parallel) computations on chunks.
  For an example of use, see tests/test_prefilter.c.

• New blosc2_set_maskout() function to avoid decompressing blocks. This
  can be handy when it is not needed to decompress all the blocks in a
  chunk. This should be always called before blosc2_decompress_ctx() and
  its effect is reset to the default (decompress all blocks) after that.

• New ALTIVEC optimizations for Power architecture. These include support
  for both shuffle and bitshuffle filters. For details, see #98.
  Thanks to Jerome Kieffer and ESRF for making this happen.

• New blosc2_frame_from_sframe() function for getting a blosc2_frame
  out of an in-memory serialized frame.

• Zstd codec updated to 1.4.4.