Changes from 2.4.3 to 2.5.0

Fixed a nasty bug that prevented retriving data correctly with large super-chunks (> 2^31 elements).
Fixed an issue in blosc2_schunk_get_slice_buffer() in the interpretation of the stop param.
Now stop is not part of the selected slice (as advertised).
Now blosc2_create_cctx() supports the same environment variables than blosc2_compress().
Now blosc2_create_dctx() supports the same environment variables than blosc2_decompress().
Added support for the split mode to be serialized in cframes/sframes.
A new splitmode field has been added to the blosc2_schunk structure.
Changed some fields in blosc2_preparams and blosc2_postparams structs:
- in -> input
- out -> output
- out_size -> output_size
- out_typesize -> output_typesize
- out_offset -> output_offset
  This was needed to allow Cython to map the fields (in is a reserved word in Python).
Disabled maskout reads in blosc2_schunk_get_slice_buffer() as they are not faster than getitem there.
Add an intermediate block size in its automatic calculation based on clevel.

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