Remove matplotlib dependency

[troyraen]

The environment created by conda-sphere_sdt.yml for the SPHEREx source discovery tool is about 1.2G. That's pretty large, which makes it more cumbersome to work with. I looked into whether we could reduce the size and found that matplotlib is being installed but not used. Removing it reduces the size of the environment by about a third, down to 770M.

I tested this on fornax and see no difference in the source discovery tool notebook when running in an environment built by the current conda-sphere_sdt.yml vs the one in this PR that removes matplotlib. I'm checking with @afaisst to make sure I'm not missing something.

[troyraen]

@afaisst confirmed that matplotlib isn't actually used and thinks it can safely be removed. He's away on travel for a couple weeks. Can you review the PR @bsipocz, or would you like me to ask someone else?

[bsipocz]

Looks good, though you may want to experiment a little bit with the base package and see how badly the installs are bloated

[bsipocz]

yes, this sounds reasonable. OTOH, I'm not sure if the users of this tool would need mpl for their work that would be beyond running the notebook as is.

A compromise could be to use the base package (called matplotlib-base) that doesn't install optional dependencies.

[troyraen]

I think users can install that if they want it. I'd prefer to keep this file limited to what's needed by the notebook.

[bsipocz]

sounds good for now.

The downside of leaving basic installs to the users is the bloated overall size -- e.g. if most of them install mpl then it would be more optimal to provide it centrally than having it take up space in most of the users directories.