Move in bpz notebooks, update paths; copy over cmnn nb changes

[OliviaLynn]

Problem & Solution Description (including issue #)

• Moving in bpz notebooks for Gather demo notebooks from older standalones #48
• Paths have been adjusted as needed
• I wasn't 100% sure on the other two data files that were stored next to the notebooks (are they also meant to be kept separate like the tar?), so I added them to the rail get-data --bpz-demo-data script in rail_base#76.

Also copies over updates to CMNN notebook made in CMNN repo to the notebook here

That was on me, should have deleted it sooner. PR is out to remove it at LSSTDESC/rail_cmnn#13

Code Quality

• My code follows the code style of this project
• I have written unit tests or justified all instances of #pragma: no cover; in the case of a bugfix, a new test that breaks as a result of the bug has been added
• My code contains relevant comments and necessary documentation for future maintainers; the change is reflected in applicable demos/tutorials (with output cleared!) and added/updated docstrings use the NumPy docstring format
• Any breaking changes, described above, are accompanied by backwards compatibility and deprecation warnings

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1de4475) 100.00% compared to head (453792d) 100.00%.

Additional details and impacted files

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  Lines            1         1           
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[sschmidt23]

BPZ_lite_with_custom_SEDs.ipynb looks good, but BPZ_lite_demo.ipynb requires one of the files from the tarball that we grab from NERSC. So, I think we need to actually insert a cell in BPZ_lite_demo.ipynb where the user can uncomment and grab those files. I can do that real quick, at which point I'll approve, but I'll wait on merging until you take a look and check that I've done it all correctly.

[sschmidt23]

Ok, I changed some text and added a commented rail get-data command, so now I think it looks good, I'll approve, if my changes look good go ahead and merge.

[OliviaLynn]

Ok, I changed some text and added a commented rail get-data command, so now I think it looks good, I'll approve, if my changes look good go ahead and merge.

Nice catch! That looks great, thank you