Adding direct model example

[kjjarvis]

Adding an example of creating a direct model into notebook 4. New Models

[github-actions]

Thank you for opening this PR. Each PR into dev requires a code review. For the code review, look at the following:

• Reviewer (someone other than author) should look for bugs, efficiency, readability, testing, and coverage in examples (if relevant).
• Ensure that each PR adding a new feature should include a test verifying that feature.
• All errors from static analysis must be resolved.
• Review the test coverage reports (if there is a change) - will be added as comment on PR if there is a change
• Review the software benchmarking results (if there is a change) - will be added as comment on PR
• Any added dependencies are included in requirements.txt, setup.py, and dev_guide.rst (this document)
• All warnings from static analysis must be reviewed and resolved - if deemed appropriate.

[codecov-commenter]

Codecov Report

Merging #101 (c78414c) into dev (cdd7d95) will decrease coverage by 0.04%.
Report is 42 commits behind head on dev.
The diff coverage is 44.44%.

@@            Coverage Diff             @@
##              dev     #101      +/-   ##
==========================================
- Coverage   84.23%   84.19%   -0.04%     
==========================================
  Files          99       99              
  Lines       10178    10183       +5     
==========================================
+ Hits         8573     8574       +1     
- Misses       1605     1609       +4     

Files Changed	Coverage Δ
src/progpy/loading/gaussian_wrapper.py	37.50% <37.50%> (-7.96%)	⬇️
src/progpy/loading/__init__.py	100.00% <100.00%> (ø)

[teubert]

Looks really great- a few suggestions