Allow deployment of prospect without DESI software stack

[weaverba137]

This PR allows the SDSS functionality of prospect to be deployed in an environment, such as NOIRLab Data Lab, that does not contain the DESI software stack.

I would like to very soon do further work to make prospect into a "real" Python package with a setup.py file and other infrastructure that would make it installable with pip.

[armengau]

I did not test the notebook in the "Data Lab" environment

[weaverba137]

That's OK, I have.

[sbailey]

ok as-is, but it seems a bit messy to have a bunch of try/except around desi imports when much of the code still requires those imports to work, thus introducing a maintenance risk about where it is ok to use desi code or not and perhaps producing non-obvious errors.

If you want to proceed with these minimally-necessary-to-work changes, ok, but consider adding unit tests to check that the things that aren't supposed to require desi code still don't. That could be done via a Travis environment that doesn't install desiutil+, or even better if there is some importlib magic that could temporarily cause import desiutil etc to not work just for those tests, even if another test had already successfully import desiutil+.

[weaverba137]

I agree that it is a bit messy right now. Data Lab at NOIRLab are pushing aggressively to release a spectrum data access service, and interactive plots with prospect is one of the end-user applications they are targeting. This branch is considered good enough to satisfy milestones associated with that.

In order to even have unit tests, we need to clean up the code and make this a real Python package, as mentioned above and in #49.