Put pixi config into pixi.toml

[ChrisBarker-NOAA]

I prefer to have the pixi config in a pixi.toml, rather than pyproject.toml.

Mostly it's aesthetic - pixi is orthogonal to the python package. -- you can use the package entirely without pixi if you want.

But also it's nice because if it's in pyproject.toml, then any changes to pyproject.toml triggers a re-build of the pixi lock file -- even if no changes to pixi -- which was very annoying early on when we were building the pyproject file -- it's pretty static now, so no biggie.

Any reason to keep them together that anyone can think of?