Change to use Pixi for package & environment management

[douglatornell]

• Initialize Pixi in pyproject.toml
• Import envs/environment-user.yaml to provide dependencies for default environment
• Drop envs/environment-user.yaml because it is no longer needed
• Create test environment and tasks based on envs/environment-test.yaml and dev docs "Running the Unit Tests" section
• Update dev docs to use Pixi tasks to run tests and produce coverage reports
• Change GitHub Actions pytest-with-coverage workflow to use reusable Pixi workflow
• Create docs environment and tasks based on envs/environment-test.yaml, envs/environment-rtd.yaml, and dev docs "Building and Previewing the Documentation" section
• Update dev docs to use Pixi tasks to build HTML docs and run link checker
• Change GitHub Actions sphinx-linkcheck workflow to use reusable Pixi workflow
• Change .readthedocs.yaml to use customized build process for Pixi from readthedocs build config docs
• Drop envs/environment-rtd.yaml because it is no longer needed
• Drop envs/environment-test.yaml because it is no longer needed
• Create dev environment based on envs/environment-dev.yaml
• Update package development docs re: the use of Pixi for package and environments management
• Drop envs/environment-dev.yaml because it is no longer needed
• Move requirements.txt to top level directory
• Add task to update requirements.txt via pip list
• Update installation docs to use Pixi
• Update use docs to pixi run reshapr ...
• Add Pixi badge to README and dev docs
• Update release process docs to use Pixi

The requirements.txt file is retained so that we will continue to get security update notifications and PRs from dependabot. It will be dropped when dependabot adds support for the pixi.lock file.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.25%. Comparing base (a910e15) to head (23c9584).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #135   +/-   ##
=======================================
  Coverage   54.25%   54.25%           
=======================================
  Files          11       11           
  Lines        1209     1209           
  Branches      110      110           
=======================================
  Hits          656      656           
  Misses        527      527           
  Partials       26       26           

Flag	Coverage Δ
unittests	54.25% <ø> (ø)

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