Add pyproject.toml, with pixi as project management tool. add pixi.lock

[jlnav]

Helps address #1444

This adds a pyproject.toml to replace setup.py, which the Python community dramatically encourages.

For project management I chose Pixi (https://pixi.sh/latest/):

• Can install/interact with both conda and PyPI packages. For example, I can specify mpich as a dev dependency without having to say "install pydantic with pip and mpich with conda".
• Can split dependencies/packages into project-inherent "environments". Use pixi install -e dev to use the dev environment. With the lockfile included we're all guaranteed to get a working environment since the lockfile was resolved earlier.
• Very fast rust backend.
• Installing a package documents/adds that install in the pyproject.toml.
• Platform-specific dependencies can be specified to only install on that platform. e.g. us linux people that install the project will get gpcam. As a macOS user I automatically get a different clang from conda-forge I use with mpicc.

This does not mean that users need Pixi, and y'all don't need it either.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.09%. Comparing base (de910fb) to head (393f9d4).
Report is 15 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff              @@
##           develop    #1459       +/-   ##
============================================
+ Coverage    78.38%   91.09%   +12.70%     
============================================
  Files           76       76               
  Lines         7680     7680               
  Branches      1354     1354               
============================================
+ Hits          6020     6996      +976     
+ Misses        1463      513      -950     
+ Partials       197      171       -26     

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[jmlarson1]

I'd check the instances of setup.py being used/referred. From the root libE directory, run

grep -ir --include=*.* "setup\.py"

[jlnav]

Noting that a very minimal setup.py has been added back. But it'll never need to be updated.

[jlnav]

More information about pixi.lock:

https://pixi.sh/latest/features/lockfile/#how-to-use-a-lock-file