Recover examples folder, update file indexing, add notebooks folder, remove legacies

[dachengx]

Following private discussion at https://xenonnt.slack.com/archives/C04JRF0AZTP/p1690266817770479.

The main idea is that GaussianModel is a well-defined and simple model to show a minimum subclass of StatisticalModel.

[github-actions]

Pull Request Test Coverage Report for Build 5681419214

• 3 of 3 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 17.964%

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Totals
Change from base Build 5680931957:	0.03%
Covered Lines:	526
Relevant Lines:	2928

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💛 - Coveralls

[github-actions]

Pull Request Test Coverage Report for Build 5681519848

• 3 of 3 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 17.964%

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Totals
Change from base Build 5680931957:	0.03%
Covered Lines:	526
Relevant Lines:	2928

---

💛 - Coveralls

[github-actions]

Pull Request Test Coverage Report for Build 5682154349

• 3 of 3 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+38.3%) to 56.257%

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Totals
Change from base Build 5680931957:	38.3%
Covered Lines:	526
Relevant Lines:	935

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💛 - Coveralls

[hammannr]

Looks good and runs! I'd suggest to get rid of legacy code with the same PR and then merge 😊

[kdund]

I concur with @hammannr , what we want from the _ - files can be copy-pasted in according to need.

[kdund]

yup