add Pixi package definition

[lucascolley]

Draft for now, I am working on being able to easily run the benchmarks provided at #92 (comment). This enables building packages that can be uploaded to a channel like https://prefix.dev/channels/lucas-forge then used on the CLI like:

❯ pixi exec -c https://prefix.dev/lucas-forge -c https://prefix.dev/conda-forge -w matplotlib -w scipy -w pyginkgo=0.0.0.build0 ./solver_pipeline.sh

Matrices directory found.
========================================
Running solver: cg
========================================
Executing: python gko_cg.py

Description

Fixes # (issue)

Type of change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules