Use Pixi

[charles-turner-1]

• Configure pyproject.toml & ci.yml for pixi.
• Add pixi task to dump a env.yml fiel to make a conda environment if wanted.

Approximately halves CI time (6 mins -> 3 mins).

TODO:

• Update docs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.10%. Comparing base (5bda03a) to head (4c00cf1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #88   +/-   ##
=======================================
  Coverage   94.10%   94.10%           
=======================================
  Files           6        6           
  Lines         424      424           
=======================================
  Hits          399      399           
  Misses         25       25           

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