Draft: Gather fsps, bpz, and cmnn demos

[OliviaLynn]

Change Description

Gathering external demos as described in #48

See running notes in my comment there.

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[codecov]

Codecov Report

All modified lines are covered by tests ✅

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[sschmidt23]

For BPZ, having the data at NERSC and just adding a curl command in the notebook seems good to me; however, BPZ_lite does expect the files to be in the SED directory, so we would still need to untar and copy them to the proper directory in the notebook. This is already done in the commented-out commands in the current demo notebook, but I'm mentioning as having them commented out as I currently would cause auto-running of the notebook to fail if rendered as part of automated testing. So, those lines should probably be uncommented when the BPZ_lite_with_custom_SEDs.ipynb notebook is moved.