feat: Add line fit example core example

[cadenmyers13]

Copied this example from https://github.com/diffpy/add2019-diffpy-cmi/tree/master.

I also tested to see how it appears when I run cmi example list and it behaves as expected:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.73%. Comparing base (656e3f3) to head (751df3c).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #51      +/-   ##
==========================================
- Coverage   81.75%   81.73%   -0.02%     
==========================================
  Files          11       11              
  Lines        1211     1210       -1     
==========================================
- Hits          990      989       -1     
  Misses        221      221              

see 1 file with indirect coverage changes

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[cadenmyers13]

@sbillinge ready for review. Adding new exampel

[sbillinge]

I think we need this example as a python module also so it will run in tests?

[cadenmyers13]

@sbillinge copied the code into a python module. This should be captured in the test

[sbillinge]

isn't that pretty! I will merge when tests pass (since they are actually testing a new example).

[cadenmyers13]

@sbillinge ready for merge. codecov is failing but it looks like maybe its from the notebook not being covered? idk 🥴