Update / cleanup the requirements

[ChrisBarker-NOAA]

What should we add?

Currently we have:

core dependencies
dev dependencies
examples dependencies

I think we should have:

core dependencies
test dependencies
docs dependencies
examples dependencies

Then "dev" dependencies would be the full set (called "all" in the pixi config now, which we could maybe keep)

Anyway, the idea is to be clear about what's needed for what, and pixi, at least, lets you set this all up very nicely.

Reference

No response