update installation check notebook

[mperrin]

Some straightforward updates to stsci_pancake_installation.ipynb:

• remove now-irrelevant outdated text for Python 2, super old versions of WebbPSF, etc.
• update target to a more sensible scene, so the output test calculation is not just noise
• show target image as well as subtracted image for test calculation

I also add a copy of the same tests to a new file jwst_pancake/tests/test_installation.py for eventual use in CI testing. See #3 :-)

The notebook output should now look like:

[york-stsci]

I can clone this repository and test out the notebook, but I haven't worked on the code in a couple of years at least. How important is my review in particular as someone who isn't a developer on this project anymore, and doesn't have any time assigned to work on it.

[mperrin]

@york-stsci your review is not at all necessary in that case!

Both the GitHub auto-suggest and @lpueyo suggested I tag you, but it sounds like that's outdated info in both cases. Cheers.