Set up environment.yml to enable notebook tests

[CommonClimate]

In response to Kim's comment, it is time we start building an environment that is specifically dedicated to the hub, so we can lighten the load for Pyleoclim and make it explicit that buying into the latter does not have to involve the rest of the pangeo(forge) ecosystem.

As Kim points out, it would need to be re-pinned once in a while (maybe quarterly), so we'll need to set up reminders for that, probably in line with Xarray releases.

As for timeline, this is not an emergency, but will definitely need to work ahead of paleoHack 4 (June 20-23 2023)