Blosc2

* Several leaks fixed. Thanks to Christoph Gohlke.

* Internal C-Blosc2 updated to 2.3.1

* Added a new `blosc2.open(urlpath, mode)` function to be able to open persisted super-chunks.

* Added a new tutorial in notebook format (`examples/tutorial-basics.ipynb`) about the basics of python-blosc2.

* Internal C-Blosc2 updated to 2.2.0

* Internal C-Blosc updated to 2.0.4.

New super-chunk implementation

* New `SChunk` class that allows to create super-chunks.
This includes the capability of storing data in 4
different ways (sparse/contiguous and in memory/on-disk),
as well as storing variable length metalayers.

* Also, during the contruction of a `SChunk` instance,
an arbitrarily large data buffer can be given so that it is
automatically split in chunks and those are appended to the
`SChunk`.

* See `examples/schunk.py` and `examples/vlmeta.py` for some examples.

* Documentation of the new API is here: https://python-blosc2.readthedocs.io

This release is the result of a grant offered by
the Python Software Foundation to Marta Iborra.
A blog entry was written describing the difficulties and relevant
aspects learned during the work:
https://www.blosc.org/posts/python-blosc2-initial-release/

* Release with C-Blosc 2.0.2 sources and binaries.

* Release with C-Blosc 2.0.1 sources and binaries.

* Headers and binaries for the C-Blosc2 library are starting
to being distributed inside wheels.

* Internal C-Blosc2 submodule updated to 2.0.0-rc2.

* Repeating measurements 4 times in benchs so as to get more
consistent figures.