xxhash algorithms

[hmaarrfk]

Closes #208

• Enables one to use xxhash algorithms if they are available.
• Provides an easy way for others to expand on the list of available algorithms.
• Choose identifiers for the xxhash methods. 'xxh128', 'xxh64', 'xxh32', 'xxh3_128', 'xxh3_64'
• Add optional dependency on xxhash
• Add instructions to https://www.fatiando.org/pooch/latest/beginner.html#hashes
• Add tests
• Add note to changelog ? Do I need to do this? Or is it autogenerated?

Tests appear as:

 pooch/tests/test_utils.py::test_xxhash_matches_strict[xxh128] ✓                       63% ██████▍   
 pooch/tests/test_utils.py::test_xxhash_matches_strict[xxh3_128] ✓                     67% ██████▋   
 pooch/tests/test_utils.py::test_xxhash_matches_strict[xxh64] ✓                        70% ███████   
 pooch/tests/test_utils.py::test_xxhash_matches_strict[xxh3_64] ✓                      73% ███████▍  
 pooch/tests/test_utils.py::test_xxhash_matches_strict[xxh32] ✓                        77% ███████▋ 

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[leouieda]

Thanks for this @hmaarrfk! This will be very handy for larger datasets. Merging it in 🚀

[leouieda]

We make the changelog as part of the release process. Editing it in every PR just ended up creating merge conflicts all the time so we gave up on that.

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[hmaarrfk]

Great thanks for the edits @leouieda