Construct more conda meta-packages

[yuvipanda]

We currently have https://github.com/conda-forge/pangeo-notebook-feedstock/ which is a metapackage providing Jupyter Runtime packages (nbgitpuller, jupyterlab, jupyterhub-singleuser, etc) required for the built docker images to run on JupyterHub / Binder.

We should create similar metapackages for:

• Visualization Libraries that are only useful in an interactive context
• Everything else in https://github.com/pangeo-data/pangeo-docker-images/blob/master/pangeo-notebook/environment.yml

This will help with two things:

1. Make sure users can install the same set of packages we have on our images locally on their machines with just conda
2. Allow us to make the forge image smaller (if needed) by not installing any of the viz libraries.

(2) can be tricky (your code might still import these unconditionally even if you are not using it) and I'm not inclined to do that unless the savings are significant. However, (1) is very helpful.

[cisaacstern]

Pangeo Forge would greatly benefit from being able to point users to a conda-installable metapackage for testing their recipes in a local environment that mirrors the worker environment (without needing to know anything about Docker). Thanks for raising this, Yuvi!