Restructure examples and imports

[alicjapolanska]

• Consolidate test cases for different models.
• Consolidate examples for flows.
• Rename legacy examples.
• Tidy up imports
• Move relevant examples utils to main utils
• Decythonize utils and explicitly round off nchains_in_val_set to an int in cross validation

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (e0daa83) 96.07% compared to head (bd3f0f5) 96.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #277 +/- ## ========================================== - Coverage 96.07% 96.00% -0.08% ========================================== Files 14 14 Lines 2318 2275 -43 ========================================== - Hits 2227 2184 -43 Misses 91 91 ```

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