Replace conda with pixi

[chuckwondo]

Dealing with conda and a conda lock file was becoming counterproductive, so this PR replaces conda and the conda lock file with pixi and pixi lock file:

1. removed all files related to creating conda-lock.yml or using the lock file or constructing the conda environment
2. moved all dependency specs into pyproject.toml, removing environment*.yml files
3. removed Makefile as it is no longer relevant, and any relevant tasks now reside in pyproject.toml under pixi sections
4. updated contributing docs
5. confirmed everything works in ADE and DPS

Fixes #101

[jjfrench]

Just a few questions, but overall things look good!