Fix reaction-diffusion example and update GP predictive

[sgreenbury]

Closes #618.

This PR:

• Fixes discrepancies with v0 reaction-diffusion example
• Revises default GP parameters to those for v0
• Adds posterior_predictive bool to GP init that enables learned observation noise to be incorporated at inference time

[review-notebook-app]

Check out this pull request on 

See visual diffs & provide feedback on Jupyter Notebooks.

---

Powered by ReviewNB

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
autoemulate/experimental/emulators/gaussian_process
exact.py					211-213
Project Total

This report was generated by python-coverage-comment-action

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.37%. Comparing base (21653a0) to head (877776b).

Files with missing lines	Patch %	Lines
...e/experimental/emulators/gaussian_process/exact.py	72.72%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #639      +/-   ##
==========================================
- Coverage   78.38%   78.37%   -0.02%     
==========================================
  Files         150      150              
  Lines       10805    10813       +8     
==========================================
+ Hits         8470     8475       +5     
- Misses       2335     2338       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[radka-j]

This looks great!